Basics

Introduction

Welcome to this C# tutorial. With the introduction of the .NET framework, Microsoft included a new language called C# (pronounced C Sharp). C# is designed to be a simple, modern, general-purpose, object-oriented programming language, borrowing key concepts from several other languages, most notably Java.

C# could theoretically be compiled to machine code, but in real life, it's always used in combination with the .NET framework. Therefore, applications written in C#, requires the .NET framework to be installed on the computer running the application. While the .NET framework makes it possible to use a wide range of languages, C# is sometimes referred to as THE .NET language, perhaps because it was designed together with the framework.

C# is an Object Oriented language and does not offer global variables or functions. Everything is wrapped in classes, even simple types like int and string, which inherits from the System.Object class.

In the following chapters, you will be guided through the most important topics about C#.

Visual C# Express

C# can be written with any text editor, like Windows Notepad, and then compiled with the C# Command line compiler, csc.exe, which comes with the .NET framework. However, most people prefer to use an IDE (Integrated Development Environment), and Microsoft offers several options for this. Their flagship is Visual Studio, which can be used to work on every possible aspect of the .NET framework. This product is very advanced, and comes in several editions. Visual Studio is not exactly cheap, and might even be too advanced for hobby programmers.

With .NET framework 2.0, Microsoft introduced the so-called Express versions, targeted at hobby programmers and people wanting to try .NET, and they continued this tradition with the later release of .NET 3.0 and 3.5. The Express versions only work for one language, like C# or VB.NET, and miss some of the really advanced features of Visual Studio. However, they are free and will work just fine for learning the languages, which is why we will use it for this tutorial.

For C# programming, you should download the Visual C# Express from http://www.microsoft.com/express/download/. Install it, and you're ready to write your first C# application!

Hello, world!

If you have ever learned a programming language, you know that they all start with the "Hello, world!" example, and who are we to break such a fine tradition? Start Visual C# Express (introduced in the last chapter), and select File -> New project… From the project dialog, select the Console application. This is the most basic application type on a Windows system, but don't worry, we won't stay here for long. Once you click Ok, Visual C# Express creates a new project for you, including a file called Program.cs. This is where all the fun is, and it should look something like this:

using System;

using System.Collections.Generic;

using System.Text;

namespace ConsoleApplication1

{

class Program

{

static void Main(string[] args)

{

}

Actually, all these lines doesn't really accomplish anything, or at least it may seem so. Try running the application by pushing F5 on your keyboard. This will make Visual C# Express compile and execute your code, but as you will see, it doesn't do much. You will likely just see a black window launch and close again. That is because our application doesn't do anything yet. In the next chapter we will go through these lines to see what they are all about, but for now, we really would like to see some results, so let's pretend that we know all about C# and add a couple of lines to get some output. Within the last set of { }, add these lines:

Console.WriteLine("Hello, world!");

Console.ReadLine();

The code of your first application should now look like this:

using System;

using System.Collections.Generic;

using System.Text;

namespace ConsoleApplication1

{

class Program

{

static void Main(string[] args)

{

Console.WriteLine("Hello, world!");

Console.ReadLine();

}

Once again, hit F5 to run it, and you will see the black window actually staying, and even displaying our greeting to the world. Okay, so we added two lines of code, but what to they do? One of the nice things about C# and the .NET framework is the fact that a lot of the code makes sense even to the untrained eye, which this example shows. The first line uses the Console class to output a line of text, and the second one reads a line of text from the console. Read? Why? Actually this is a bit of a trick, since without it, the application would just end and close the window with the output before anyone could see it. The ReadLine command tells the application to wait for input from the user, and as you will notice, the console window now allows you to enter text. Press Enter to close it. Congratulations, you have just created your first C# application! Read on in the next chapter for even more information about what's actually going on.

Hello world explained

In the previous chapter, we tried writing a piece of text to the console, in our first C# application. To see some actual progress, we didn't go into much detail about the lines of code we used, so this chapter is an explanation of the Hello world example code. As you can probably see from the code, some of the lines look similar, so we will bring them back in groups for an individual explanation. Let's start with the shortest and most common characters in our code: The { and }. They are often referred to as curly braces, and in C#, they mark the beginning and end of a logical block of code. The curly braces are used in lots of other languages, including C++, Java, JavaScript and many others. As you can see in the code, they are used to wrap several lines of code which belongs together. In later examples, it will be clearer how they are used.

Now let's start from the beginning:

using System;

using System.Collections.Generic;

using System.Text;

using is a keyword, highlighted with blue by the editor. The using keyword imports a namespace, and a namespace is a collection of classes. Classes brings us some sort of functionality, and when working with an advanced IDE like Visual C# Express, it will usually create parts of the trivial code for us. In this case, it created a class for us, and imported the namespaces which is required or expected to be used commonly. In this case, 3 namespaces are imported for us, each containing lots of useful classes. For instance, we use the Console class, which is a part of the System namespace.

As you can see, we even get our own namespace:

namespace ConsoleApplication1

The namespace ConsoleApplication1 is now the main namespace for this application, and new classes will be a part of it by default. Obviously, you can change this, and create classes in another namespace. In that case, you will have to import this new namespace to use it in your application, with the using statement, like any other namespace.

Next, we define our class. Since C# is truly an Object Oriented language, every line of code that actually does something, is wrapped inside a class. In the case, the class is simply called Program:

class Program

We can have more classes, even in the same file. For now, we only need one class. A class can contain several variables, properties and methods, concepts we will go deeper into later on. For now, all you need to know is that our current class only contains one method and nothing else. It's declared like this:

static void Main(string[] args)

This line is probably the most complicated one in this example, so let's split it up a bit. The first word is static. The static keyword tells us that this method should be accesible without instantiating the class, but more about this in our chapter about classes. The next keyword is void, and tells us what this method should return. For instance, int could be an integer or a string of text, but in this case, we don't want our method to return anything, or void, which is the same as no type. The next word is Main, which is simply the name of our method. This method is the so-called entry-point of our application, that is, the first piece of code to be executed, and in our example, the only piece to be executed. Now, after the name of a method, a set of arguments can be specified within a set of parentheses. In our example, our method takes only one argument, called args. The type of the argument is a string, or to be more precise, an array of strings, but more on that later. If you think about it, this makes perfect sense, since Windows applications can always be called with an optinal set of arguments. These arguments will be passed as text strings to our main method.

And that's it. You should now have a basic understanding of our first C# application, as well as the basic principles of what makes a console application work.

Data types

Data types are used everywhere in a programming language like C#. Because it's a strongly typed language, you are required to inform the compiler about which data types you wish to use every time you declare a variable, as you will see in the chapter about variables. In this chapter we will take a look at some of the most used data types and how they work.

bool is one of the simplest data types. It can contain only 2 values - false or true. The bool type is important to understand when using logical operators like the if statement.

int is short for integer, a data type for storing numbers without decimals. When working with numbers, int is the most commonly used data type. Integers have several data types within C#, depending on the size of the number they are supposed to store.

string is used for storing text, that is, a number of chars. In C#, strings are immutable, which means that strings are never changed after they have been created. When using methods which changes a string, the actual string is not changed - a new string is returned instead.

char is used for storing a single character.

float is one of the data types used to store numbers which may or may not contain decimals.

Variables

A variable can be compared to a storage room, and is essential for the programmer. In C#, a variable is declared like this:

<data type> <name>;

An example could look like this:

string name;

That's the most basic version. Usually, you wish to assign a visibility to the variable, and perhaps assign a value to it at the same time. It can be done like this:

<visibility> <data type> <name> = <value>;

And with an example:

private string name = "John Doe";

The visibility part is explained elsewhere in this tutorial, so let's concentrate on the variable part. We will jump straight to an example of actually using a couple of them:

using System;

namespace ConsoleApplication1

{

class Program

{

static void Main(string[] args)

{

string firstName = "John";

string lastName = "Doe";

Console.WriteLine("Name: " + firstName + " " + lastName);

Console.WriteLine("Please enter a new first name:");

firstName = Console.ReadLine();

Console.WriteLine("New name: " + firstName + " " + lastName);

Console.ReadLine();

}

Okay, a lot of this has already been explained, so we will jump directly to the interesting part. First of all, we declare a couple of variables of the string type. A string simply contains text, as you can see, since we give them a value straight away. Next, we output a line of text to the console, where we use the two variables. The string is made up by using the + characters to "collect" the different parts.

Next, we urge the user to enter a new first name, and then we use the ReadLine() method to read the user input from the console and into the firstName variable. Once the user presses the Enter key, the new first name is assigned to the variable, and in the next line we output the name presentation again, to show the change. We have just used our first variable and the single most important feature of a variable: The ability to change its value at runtime.

Another interesting example is doing math. Here is one, based on a lot of the same code we have just used:

int number1, number2;

Console.WriteLine("Please enter a number:");

number1 = int.Parse(Console.ReadLine());

Console.WriteLine("Thank you. One more:");

number2 = int.Parse(Console.ReadLine());

Console.WriteLine("Adding the two numbers: " + (number1 + number2));

Console.ReadLine();

Put this in our Main method, and try it out. The only new "trick" we use here, is the int.Parse() method. It simply reads a string and converts it into an integer. As you can see, this application makes no effort to validate the user input, and if you enter something which is not a number, an exception will be raised. More about those later.

The if statement

One of the single most important statements in every programming language is the if statement. Being able to set up conditional blocks of code is a fundamental principal of writing software. In C#, the if statement is very simple to use. If you have already used another programming language, chances are that you can use the if statement of C# straight away. In any case, read on to see how it's used. The if statement needs a boolean result, that is, true or false. In some programming languages, several datatypes can be automatically converted into booleans, but in C#, you have to specifically make the result boolean. For instance, you can't use if(number), but you can compare number to something, to generate a true or false, like we do later on.

In the previous chapter we looked at variables, so we will expand on one of the examples to see how conditional logic can be used.

using System;

namespace ConsoleApplication1

{

class Program

{

static void Main(string[] args)

{

int number;

Console.WriteLine("Please enter a number between 0 and 10:");

number = int.Parse(Console.ReadLine());

if(number > 10)

Console.WriteLine("Hey! The number should be 10 or less!");

else

if(number < 0)

Console.WriteLine("Hey! The number should be 0 or more!");

else

Console.WriteLine("Good job!");

Console.ReadLine();

}

We use 2 if statements to check if the entered number is between 0 and 10, and a companion of the if statement: The else keyword. Its meaning should be obvious to anyone speaking English - it simply offers an alternative to the code being executed if the condition of the if statement is not met.

As you may have noticed, we don't use the { and } characters to define the conditional blocks of code. The rule is that if a block only contains a single line of code, the block characters are not required. Now, this seems like a lot of lines to simply check a number, doesn't it? It can be done with fewer lines of code, like this:

if((number > 10) || (number < 0))

Console.WriteLine("Hey! The number should be 0 or more and 10 or less!");

else

Console.WriteLine("Good job!");

We put each condition in a set of parentheses, and then we use the || operator, which simply means "or", to check if the number is either more than 10 OR less than 0. Another operator you will be using a lot is the AND operator, which is written like this: &&. Could we have used the AND operator instead? Of course, we simply turn it around a bit, like this:

if((number <= 10) && (number >= 0))

Console.WriteLine("Good job!");

else

Console.WriteLine("Hey! The number should be 0 or more and 10 or less!");

This introduces a couple of new operators, the "less than or equal too" and the "bigger than or equal too".

The switch statement

The switch statement is like a set of if statements. It's a list of possibilities, with an action for each possibility, and an optional default action, in case nothing else evaluates to true. A simple switch statement looks like this:

int number = 1;

switch(number)

{

case 0:

Console.WriteLine("The number is zero!");

break;

case 1:

Console.WriteLine("The number is one!");

break;

}

The identifier to check is put after the switch keyword, and then there's the list of case statements, where we check the identifier against a given value. You will notice that we have a break statement at the end of each case. C# simply requires that we leave the block before it ends. In case you were writing a function, you could use a return statement instead of the break statement.

In this case, we use an integer, but it could be a string too, or any other simple type. Also, you can specify the same action for multiple cases. We will do that in the next example too, where we take a piece of input from the user and use it in our switch statement:

Console.WriteLine("Do you enjoy C# ? (yes/no/maybe)");

string input = Console.ReadLine();

switch(input.ToLower())

{

case "yes":

case "maybe":

Console.WriteLine("Great!");

break;

case "no":

Console.WriteLine("Too bad!");

break;

}

In this example, we ask the user a question, and suggest that they enter either yes, no or maybe. We then read the user input, and create a switch statement for it. To help the user, we convert the input to lowercase before we check it against our lowercase strings, so that there is no difference between lowercase and uppercase letters. Still, the user might make a typo or try writing something completely different, and in that case, no output will be generated by this specific switch statement. Enter the default keyword!

Console.WriteLine("Do you enjoy C# ? (yes/no/maybe)");

string input = Console.ReadLine();

switch(input.ToLower())

{

case "yes":

case "maybe":

Console.WriteLine("Great!");

break;

case "no":

Console.WriteLine("Too bad!");

break;

default:

Console.WriteLine("I'm sorry, I don't understand that!");

break;

}

If none of the case statements has evaluated to true, then the default statement, if any, will be executed. It is optional, as we saw in the previous examples.

Loops

Another essential technique when writing software is looping - the ability to repeat a block of code X times. In C#, they come in 4 different variants, and we will have a look at each one of them.

The while loop

The while loop is probably the most simple one, so we will start with that. The while loop simply executes a block of code as long as the condition you give it is true. A small example, and then some more explanation:

using System;

namespace ConsoleApplication1

    class Program

        static void Main(string[] args)

            int number = 0;

            while(number < 5)

                Console.WriteLine(number);

                number = number + 1;

            Console.ReadLine();

Try running the code. You will get a nice listing of numbers, from 0 to 4. The number is first defined as 0, and each time the code in the loop is executed, it's incremented by one. But why does it only get to 4, when the code says 5? For the condition to return true, the number has to be less than 5, which in this case means that the code which outputs the number is not reached once the number is equal to 5. This is because the condition of the while loop is evaluated before it enters the code block.

The do loop

The opposite is true for the do loop, which works like the while loop in other aspects through. The do loop evaluates the condition after the loop has executed, which makes sure that the code block is always executed at least once.

do

    Console.WriteLine(number);

    number = number + 1;

} while(number < 5);

The output is the same though - once the number is more than 5, the loop is exited.

The for loop

The for loop is a bit different. It's preferred when you know how many iterations you want, either because you know the exact amount of iterations, or because you have a variable containing the amount. Here is an example on the for loop.

using System;

namespace ConsoleApplication1

    class Program

        static void Main(string[] args)

            int number = 5;

            for(int i = 0; i < number; i++)

                Console.WriteLine(i);

            Console.ReadLine();

This produces the exact same output, but as you can see, the for loop is a bit more compact. It consists of 3 parts - we initialize a variable for counting, set up a conditional statement to test it, and increment the counter (++ means the same as "variable = variable + 1"). The first part, where we define the i variable and set it to 0, is only executed once, before the loop starts. The last 2 parts are executed for each iteration of the loop. Each time, i is compared to our number variable - if i is smaller than number, the loop runs one more time. After that, i is increased by one. Try running the program, and afterwards, try changing the number variable to something bigger or smaller than 5. You will see the loop respond to the change.

The foreach loop

The last loop we will look at, is the foreach loop. It operates on collections of items, for instance arrays or other built-in list types. In our example we will use one of the simple lists, called an ArrayList. It works much like an array, but don't worry, we will look into it in a later chapter.

using System;

using System.Collections;

namespace ConsoleApplication1

    class Program

        static void Main(string[] args)

            ArrayList list = new ArrayList();

            list.Add("John Doe");

            list.Add("Jane Doe");

            list.Add("Someone Else");

            foreach(string name in list)

                Console.WriteLine(name);

            Console.ReadLine();

Okay, so we create an instance of an ArrayList, and then we add some string items to it. We use the foreach loop to run through each item, setting the name variable to the item we have reached each time. That way, we have a named variable to output. As you can see, we declare the name variable to be of the string type – you always need to tell the foreach loop which datatype you are expecting to pull out of the collection. In case you have a list of various types, you may use the object class instead of a specific class, to pull out each item as an object. When working with collections, you are very likely to be using the foreach loop most of the time, mainly because it’s simpler than any of the other loops for these kind of operations.

Functions

A function allows you to encapsulate a piece of code and call it from other parts of your code. You may very soon run into a situation where you need to repeat a piece of code, from multiple places, and this is where functions come in. In C#, they are basically declared like this:

<visibility> <return type> <name>(<parameters>)

{

}

To call a function, you simply write its name, an open parenthesis, then parameters, if any, and then a closing parenthesis, like this:

DoStuff();

Here is an example of our DoStuff() function:

public void DoStuff()

{

Console.WriteLine("I'm doing something...");

}

The first part, public, is the visibility, and is optional. If you don't define any, then the function will be private. More about that later on.
Next is the type to return. It could be any valid type in C#, or as we have done it here, void. A void means that this function returns absolutely nothing. Also, this function takes no parameters, as you can see from the empty set of parentheses, so it's actually just a tad bit boring. Let's change that:

public int AddNumbers(int number1, int number2)

{

int result = number1 + number2;

return result;

}

We've changed almost everything. The function now returns an integer, it takes two parameters (both integers), and instead of outputting something, it makes a calculation and then returns the result. This means that we can add two numbers from various places in our code, simply by calling this function, instead of having to write the calculation code each time. While we don't save that much time and effort in this small example, you better believe that you will learn to love functions, the more you use C#. This function is called like this:

int result = AddNumbers(10, 5);

Console.WriteLine(result);

As mentioned, this function actually returns something, and it has to, because we told C# that it's supposed to do so. When declaring anything else than void as a return type, we are forcing our self to return something. You can try removing the return line from the example above, and see the compiler complain:

'AddNumbers(int, int)': not all code paths return a value

The compiler is reminding us that we have a function which doesn't return something, although we promised. And the compiler is pretty clever! Instead of removing the line, try something like this:

public int AddNumbers(int number1, int number2)

{

int result = number1 + number2;

if(result > 10)

{

return result;

}

You will see the exact same error - but why? Because there is no guarantee that our if statement will evaluate to true and the return line being executed. You can solve this by having a second, default like return statement in the end:

public int AddNumbers(int number1, int number2)

{

int result = number1 + number2;

if(result > 10)

{

return result;

}

return 0;

}

This will fix the problem we created for our self, and it will also show you that we can have more than one return statement in our function. As soon as a return statement is reached, the function is left and no more code in it is executed. In this case, it means that as long as the result is higher than 10, the "return 0" is never reached.

Function parameters

In the previous chapter, we had a look at functions. We briefly discussed parameters, but only briefly. While parameters are very simple and straight forward to use, there are tricks which can make them a lot more powerful.

The first thing that we will take a look at, is the out and ref modifiers. C#, and other languages as well, differ between two parameters: "by value" and "by reference". The default in C# is "by value", which basically means that when you pass on a variable to a function call, you are actually sending a copy of the object, instead of a reference to it. This also means that you can make changes to the parameter from inside the function, without affecting the original object you passed as a parameter.

With the ref and the out keyword, we can change this behavior, so we pass along a reference to the object instead of its value.

The ref modifier

Consider the following example:

static void Main(string[] args)

    int number = 20;

    AddFive(number);

    Console.WriteLine(number);

    Console.ReadKey();

static void AddFive(int number)

    number = number + 5;

We create an integer, assign the number 20 to it, and then we use the AddFive() method, which should add 5 to the number. But does it? No. The value we assign to number inside the function, is never carried out of the function, because we have passed a copy of the number value instead of a reference to it. This is simply how C# works, and in a lot of cases, it's the preferred result. However, in this case, we actually wish to modify the number inside our function. Enter the ref keyword:

static void Main(string[] args)

    int number = 20;

    AddFive(ref number);

    Console.WriteLine(number);

    Console.ReadKey();

static void AddFive(ref int number)

    number = number + 5;

As you can see, all we've done is adding the ref keyword to the function declaration as well as the call to the function. If you run the program now, you will see that the value of number has now changed, once we return from the function call.

The out modifier

The out modifier works pretty much like the ref modifier. They both ensure that the parameter is passed by reference instead of by value, but they do come with two important differences: A value passed to a ref modifier has to be initialized before calling the method - this is not true for the out modifier, where you can use un-initialized values. On the other hand, you can't leave a function call with an out parameter, without assigning a value to it. Since you can pass in un-initialized values as an out parameter, you are not able to actually use an out parameter inside a function - you can only assign a new value to it.

Whether to use out or ref really depends on the situation, as you will realize once you start using them. Both are typically used to work around the issue of only being able to return one value from a function, with C#.

Using the out modifier is just like using the ref modifier, as shown above. Simply change the ref keyword to the out keyword.

The params modifier

So far, all of our functions have accepted a fixed amount of parameters. However, in some cases, you might need a function which takes an arbitrary number of parameters. This could of course be done by accepting an array or a list as a parameter, like this:

static void GreetPersons(string[] names) { }

However, calling it would be a bit clumsy. In the shortest form, it would look like this:

GreetPersons(new string[] { "John", "Jane", "Tarzan" });

It is acceptable, but it can be done even smarter, with the params keyword:

static void GreetPersons(params string[] names) { }

Calling it would then look like this:

GreetPersons("John", "Jane", "Tarzan");

Another advantage of using the params approach, is that you are allowed to pass zero parameters to it as well.
Functions with params can even take other parameters as well, as long as the parameter with the params keyword are the last one. Besides that, only one parameter using the params keyword can be used per function. Here is a last and more complete example:

static void Main(string[] args)

    GreetPersons(0);

    GreetPersons(25, "John", "Jane", "Tarzan");

    Console.ReadKey();

static void GreetPersons(int someUnusedParameter, params string[] names)

    foreach(string name in names)

        Console.WriteLine("Hello, " + name);

Arrays

Arrays works as collections of items, for instance strings. You can use them to gather items in a single group, and perform various operations on them, e.g. sorting. Besides that, several methods within the framework work on arrays, to make it possible to accept a range of items instead of just one. This fact alone makes it important to know a bit about arrays.

Arrays are declared much like variables, with a set of [] brackets after the datatype, like this:

string[] names;

You need to instantiate the array to use it, which is done like this:

string[] names = new string[2];

The number (2) is the size of the array, that is, the amount of items we can put in it. Putting items into the array is pretty simple as well:

names[0] = "John Doe";

But why 0? As it is with so many things in the world of programming, the counting starts from 0 instead of 1. So the first item is indexed as 0, the next as 1 and so on. You should remember this when filling the array with items, because overfilling it will cause an exception. When you look at the initializer, setting the array to a size of 2, it might seem natural to put item number 0, 1 and 2 into it, but this is one item too much. If you do it, an exception will be thrown. We will discuss exceptions in a later chapter.

Earlier, we learned about loops, and obviously these go great with arrays. The most common way of getting data out of an array, is to loop through it and perform some sort of operation with each value. Let's use the array from before, to make a real example:

using System;

using System.Collections;

namespace ConsoleApplication1

{

class Program

{

static void Main(string[] args)

{

string[] names = new string[2];

names[0] = "John Doe";

names[1] = "Jane Doe";

foreach(string s in names)

Console.WriteLine(s);

Console.ReadLine();

}

We use the foreach loop, because it's the easiest, but of course we could have used one of the other types of loop instead. The for loop is good with arrays as well, for instance if you need to count each item, like this:

for(int i = 0; i < names.Length; i++)

Console.WriteLine("Item number " + i + ": " + names[i]);

It's actually very simple. We use the Length property of the array to decide how many times the loop should iterate, and then we use the counter (i) to output where we are in the process, as well as get the item from the array. Just like we used a number, a so called indexer, to put items into the array, we can use it to get a specific item out again.

I told you earlier that we could use an array to sort a range of values, and it's actually very easy. The Array class contains a bunch of smart methods for working with arrays. This example will use numbers instead of strings, just to try something else, but it could just as easily have been strings. I wish to show you another way of populating an array, which is much easier if you have a small, predefined set of items that you wish to put into your array. Take a look:

int[] numbers = new int[5] { 4, 3, 8, 0, 5 };

With one line, we have created an array with a size of 5, and filled it with 5 integers. By filling the array like this, you get an extra advantage, since the compiler will check and make sure that you don't put too many items into the array. Try adding a number more - you will see the compiler complain about it.

Actually, it can be done even shorter, like this:

int[] numbers = { 4, 3, 8, 0, 5 };

This is short, and you don't have to specify a size. The first approach may be easier to read later on though.

Let's try sorting the array - here's a complete example:

using System;

using System.Collections;

namespace ConsoleApplication1

{

class Program

{

static void Main(string[] args)

{

int[] numbers = { 4, 3, 8, 0, 5 };

Array.Sort(numbers);

foreach(int i in numbers)

Console.WriteLine(i);

Console.ReadLine();

}

The only real new thing here is the Array.Sort command. It can take various parameters, for various kinds of sorting, but in this case, it simply takes our array. As you can see from the result, our array has been sorted. The Array class has other methods as well, for instance the Reverse() method. You can look it up in the documentation to see all the features of the Array class.

The arrays we have used so far have only had one dimension. However, C# arrays can be multidimensional, sometimes referred to as arrays in arrays. Multidimensional arrays come in two flavors with C#: Rectangular arrays and jagged arrays. The difference is that with rectangular arrays, all the dimensions have to be the same size, hence the name rectangular. A jagged array can have dimensions of various sizes. Multidimensional arrays are a heavy subject, and a bit out of the scope of this tutorial.

Classes:

Introduction to C# classes

In lots of programming tutorials, information about classes will be saved for much later. However, since C# is all about Object Oriented programming and thereby classes, we will make a basic introduction to the most important stuff already now.

First of all, a class is a group of related methods and variables. A class describes these things, and in most cases, you create an instance of this class, now referred to as an object. On this object, you use the defined methods and variables. Of course, you can create as many instances of your class as you want to. Classes, and Object Oriented programming in general, is a huge topic. We will cover some of it in this chapter as well as in later chapters, but not all of it.

In the Hello world chapter, we saw a class used for the first time, since everything in C# is built upon classes. Let's expand our Hello world example with a class we built on our own.

using System;

namespace ConsoleApplication1

{

class Program

{

static void Main(string[] args)

{

Car car;

car = new Car("Red");

Console.WriteLine(car.Describe());

car = new Car("Green");

Console.WriteLine(car.Describe());

Console.ReadLine();

}

class Car

{

private string color;

public Car(string color)

{

this.color = color;

}

public string Describe()

{

return "This car is " + Color;

}

public string Color

{

get { return color; }

set { color = value; }

}

Okay, lots of new stuff here, but almost all of it is based on stuff we've already used earlier in this tutorial. As you can see, we have defined a new class, called Car. It's declared in the same file as our main application, for an easier overview, however, usually new classes are defined in their own files. It defines a single variable, called color, which of course is used to tell the color of our car. We declared it as private, which is good practice - accessing variables from the outside should be done using a property. The Color property is defined in the end of the class, giving access to the color variable.

Besides that, our Car class defines a constructor. It takes a parameter which allows us to initialize Car objects with a color. Since there is only one constructor, Car objects can only be instantiated with a color. The Describe() method allows us to get a nice message with the single piece of information that we record about our. It simply returns a string with the information we provide.

Now, in our main application, we declare a variable of the type Car. After that, we create a new instance of it, with "Red" as parameter. According to the code of our class, this means that the color red will be assigned as the color of the car. To verify this, we call the Describe() method, and to show how easy we can create several instances of the same class, we do it again, but with another color. We have just created our first functional class and used it.

In the following chapters, concepts like properties, constructors and visibility will be explained in more depth.

Properties

Properties allow you to control the accessibility of a classes variables, and is the recommended way to access variables from the outside in an object oriented programming language like C#. In our chapter on classes, we saw the use of a property for the first time, and the concept is actually quite simple. A property is much like a combination of a variable and a method - it can't take any parameters, but you are able to process the value before it's assigned to our returned. A property consists of 2 parts, a get and a set method, wrapped inside the property:

private string color;

public string Color

{

get { return color; }

set { color = value; }

}

The get method should return the variable, while the set method should assign a value to it. Our example is as simple as it gets, but it can be extended. Another thing you should know about properties is the fact that only one method is required - either get or set, the other is optional. This allows you to define read-only and write-only properties. Here is a better example of why properties are useful:

public string Color

{

get

{

return color.ToUpper();

}

set

{

if(value == "Red")

color = value;

else

Console.WriteLine("This car can only be red!");

}

Okay, we have just made our property a bit more advanced. The color variable will now be returned in uppercase characters, since we apply the ToUpper() method to it before returning it, and when we try to set the color, only the value "Red" will be accepted. Sure, this example is not terrible useful, but it shows the potential of properties.

Constructors and destructors

Constructors are special methods, used when instantiating a class. A constructor can never return anything, which is why you don't have to define a return type for it. A normal method is defined like this:

public string Describe()

A constructor can be defined like this:

public Car()

In our example for this chapter, we have a Car class, with a constructor which takes a string as argument. Of course, a constructor can be overloaded as well, meaning we can have several constructors, with the same name, but different parameters. Here is an example:

public Car()

public Car(string color)

    this.color = color;

A constructor can call another constructor, which can come in handy in several situations. Here is an example:

public Car()

    Console.WriteLine("Constructor with no parameters called!");

public Car(string color) : this()

    this.color = color;

    Console.WriteLine("Constructor with color parameter called!");

If you run this code, you will see that the constructor with no parameters is called first. This can be used for instantiating various objects for the class in the default constructor, which can be called from other constructors from the class. If the constructor you wish to call takes parameters, you can do that as well. Here is a simple example:

public Car(string color) : this()

    this.color = color;

    Console.WriteLine("Constructor with color parameter called!");

public Car(string param1, string param2) : this(param1)

If you call the constructor which takes 2 parameters, the first parameter will be used to invoke the constructor that takes 1 parameter.

Destructors

Since C# is garbage collected, meaing that the framework will free the objects that you no longer use, there may be times where you need to do some manual cleanup. A destructor, a method called once an object is disposed, can be used to cleanup resources used by the object. Destructors doesn't look very much like other methods in C#. Here is an example of a destructor for our Car class:

~Car()

    Console.WriteLine("Out..");

Once the object is collected by the garbage collector, this method is called.

Method overloading

A lot of programming languages supports a technique called default/optional parameters. It allows the programmer to make one or several parameters optional, by giving them a default value. It's especially practical when adding functionality to existing code. For instance, you may wish to add functionality to an existing function, which requires one or more parameters to be added. By doing so, you would break existing code calling this function, since they would now not be passing the required amount of parameters. To work around this, you could define the newly added parameters as optional, and give them a default value that corresponds to how the code would work before adding the parameters.

As of writing this, C# does not support default parameters. They have been announced for C# version 4.0, but up until that, C# coders have been using a different technique, which basically does the same, called method overloading. It allows the programmer do define several methods with the same name, as long as they take a different set of parameters. When you use the classes of the .NET framework, you will soon realize that method overloading is used all over the place. A good example of this, is the Substring() method of the String class. It is with an extra overload, like this:

string Substring (int startIndex)

string Substring (int startIndex, int length)

You can call it with either one or two parameters. If you only call it with one parameter, the length parameter is assumed to be the rest of the string, saving us time whenever we simply want to get the last part of a string.

So, by defining several versions of the same function, how do we avoid having the same code several places? It's actually quite simple: We let the simple versions of the method make the complex version of it do all the work. Consider the following example:

class SillyMath

{

public static int Plus(int number1, int number2)

{

return Plus(number1, number2, 0);

}

public static int Plus(int number1, int number2, int number3)

{

return number1 + number2 + number3;

}

We define a Plus method, in two different versions. The first one takes two paramaters, for adding two numbers, while the second version takes three numbers. The actual work is done in the version that takes three numbers - if we only wish to add two, we call the three parameter version, and simply use 0 as the third paramater, acting as a default value. I know, I know, it's a silly example, as indicated by the name of the class, but it should give you an idea about how it all works. Now, whenever you feel like doing advanced math by adding a total of four numbers (just kidding here), it's very simple to add a new overload:

class SillyMath

{

public static int Plus(int number1, int number2)

{

return Plus(number1, number2, 0);

}

public static int Plus(int number1, int number2, int number3)

{

return Plus(number1, number2, number3, 0);

}

public static int Plus(int number1, int number2, int number3, int number4)

{

return number1 + number2 + number3 + number4;

}

The cool thing about this, is that all your existing calls to the Plus method will continue working, as if nothing had been changed. The more you use C#, the more you will learn to appreciate method overloading.

Visibility

The visibility of a class, a method, a variable or a property tells us how this item can be accessed. The most common types of visibility are private and public, but there are actually several other types of visibility within C#. Here is a complete list, and although some of them might not feel that relevant to you right now, you can always come back to this page and read up on them:

public - the member can be reached from anywhere. This is the least restrictive visibility. Enums and interfaces are, by default, publicly visible.

protected - members can only be reached from within the same class, or from a class which inherits from this class.

internal - members can be reached from within the same project only.

protected internal - the same as internal, except that also classes which inherits from this class can reach it members, even from another project.

private - can only be reached by members from the same class. This is the most restrictive visibility. Classes and structs are by default set to private visibility.

So for instance, if you have two classes, Class1 and Class2, private members from Class1 can only be used within Class1. You can't create a new instance of Class1 inside of Class2, and then expect to be able to use its private members.

If Class2 inherits from Class1, then only non-private members can be reached from inside of Class2.

Static members

As we saw in a previous chapter, the usual way to communicate with a class, is to create a new instance of the class, and then work on the resulting object. In most cases, this is what classes are all about - the ability to instantiate multiple copies of the same class and then use them differently in some way. However, in some cases, you might like to have a class which you may use without instantiating it, or at least a class where you can use members of it without creating an object for it. For instance, you may have a class with a variable that always remains the same, no matter where and how it's used. This is called a static member, static because it remains the same.

A class can be static, and it can have static members, both functions and fields. A static class can't be instantiated, so in other words, it will work more as a grouping of related members than an actual class. You may choose to create a non-static class instead, but let it have certain static members. A non-static class can still be instantiated and used like a regular class, but you can't use a static member on an object of the class. A static class may only contain static members.

First, here is an example of a static class:

public static class Rectangle

{

public static int CalculateArea(int width, int height)

{

return width * height;

}

As you can see, we use the static keyword to mark the class as static, and then we use it again to mark the method, CalculateArea, as static as well. If we didn't do that, the compiler would complain, since we can't have a non-static member of a static class.

To use this method, we call it directly on the class, like this:

Console.WriteLine("The area is: " + Rectangle.CalculateArea(5, 4));

We could add other helpful methods to the Rectangle class, but perhaps you are wondering why we are passing on width and height to the actual method, instead of storing it inside the class and then pulling them from there when needed? Because it's static! We could store them, but only one set of dimensions, because there is only one version of a static class. This is very important to understand.

Instead, we can make the class non-static, and then have the CalculateArea as a utility function on this class:

public class Rectangle

{

private int width, height;

public Rectangle(int width, int height)

{

this.width = width;

this.height = height;

}

public void OutputArea()

{

Console.WriteLine("Area output: " + Rectangle.CalculateArea(this.width, this.height));

}

public static int CalculateArea(int width, int height)

{

return width * height;

}

As you can see, we have made the class non-static. We have also added a constructor, which takes a width and a height and assigns it to the instance. Then we have added an OutputArea method, which uses the static method to calculate the area. This is a fine example of mixing static members with non-static members, in a non-static class.

A common usage of static classes, although frowned upon by some people, are utility/helper classes, where you collect a bunch of useful methods, which might not belong together, but doesn't really seem to fit elsewhere either.

Inheritance

One of the absolute key aspects of Object Oriented Programming (OOP), which is the concept that C# is built upon, is inheritance, the ability to create classes which inherits certain aspects from parent classes. The entire .NET framework is built on this concept, with the "everything is an object" as a result of it. Even a simple number is an instance of a class, which inherits from the System.Object class, although .NET helps you out a bit, so you can assign a number directly, instead of having to create a new instance of e.g. the integer class.

This subject can be a bit difficult to comprehend, but sometimes it help with some examples, so let's start with a simple one of those:

public class Animal

{

public void Greet()

{

Console.WriteLine("Hello, I'm some sort of animal!");

}

public class Dog : Animal

{

}

First, we define an Animal class, with a simple method to output a greeting. Then we define a Dog class, and with a colon, we tell C# that the Dog class should inherit from the Animal class. The beautiful thing about this is that it makes sense in the real world as well - a Dog is, obviously, an Animal. Let's try using the classes:

Animal animal = new Animal();

animal.Greet();

Dog dog = new Dog();

dog.Greet();

If you run this example, you will notice that even though we have not defined a Greet() method for the Dog class, it still knows how to greet us, because it inherits this method from the Animal class. However, this greeting is a bit anonymous, so let's customize it when we know which animal it is:

public class Animal

{

public virtual void Greet()

{

Console.WriteLine("Hello, I'm some sort of animal!");

}

public class Dog : Animal

{

public override void Greet()

{

Console.WriteLine("Hello, I'm a dog!");

}

Besides the added method on the Dog class, you should notice two things: I have added the virtual keyword to the method on the Animal class, and on the Dog class, I use the override keyword. In C#, you are not allowed to override a member of a class unless it's marked as virtual. If you want to, you can still access the inherited method, even when you override it, using the base keyword.

public override void Greet()

{

base.Greet();

Console.WriteLine("Yes I am - a dog!");

}

Methods is not the only thing to get inherited, though. In fact, pretty much all class members will be inherited, including fields and properties. Just remember the rules of visibilty, as discussed in a previous chapter.

Inheritance is not only from one class to another - you can have a whole hierarchy of classes, which inherits from eachother. For instance, we could create a Puppy class, which inherits from our Dog class, which in turn inherits from the Animal class. What you can't do in C#, is to let one class inherit from several other classes at the same time. Multiple inheritance, as it's called, is not supported by C#.

Abstract classes

Abstract classes, marked by the keyword abstract in the class definition, are typically used to define a base class in the hierarchy. What's special about them, is that you can't create an instance of them - if you try, you will get a compile error. Instead, you have to subclass them, as taught in the chapter on inheritance, and create an instance of your subclass. So when do you need an abstract class? It really depends on what you do. To be honest, you can go a long way without needing an abstract class, but they are great for specific things, like frameworks, which is why you will find quite a bit of abstract classes within the .NET framework it self. A good rule of thumb is that the name actually makes really good sense - abstract classes are very often, if not always, used to describe something abstract, something that is more of a concept than a real thing.

In this example, we will create a base class for four legged animals and then create a Dog class, which inherits from it, like this:

namespace AbstractClasses

{

class Program

{

static void Main(string[] args)

{

Dog dog = new Dog();

Console.WriteLine(dog.Describe());

Console.ReadKey();

}

abstract class FourLeggedAnimal

{

public virtual string Describe()

{

return "Not much is known about this four legged animal!";

}

class Dog : FourLeggedAnimal

{

}

If you compare it with the examples in the chapter about inheritance, you won't see a big difference. In fact, the abstract keyword in front of the FourLeggedAnimal definition is the biggest difference. As you can see, we create a new instance of the Dog class and then call the inherited Describe() method from the FourLeggedAnimal class. Now try creating an instance of the FourLeggedAnimal class instead:

FourLeggedAnimal someAnimal = new FourLeggedAnimal();

You will get this fine compiler error:

Cannot create an instance of the abstract class or interface 'AbstractClasses.FourLeggedAnimal'

Now, as you can see, we just inherited the Describe() method, but it isn't very useful in it's current form, for our Dog class. Let's override it:

class Dog : FourLeggedAnimal

{

public override string Describe()

{

return "This four legged animal is a Dog!";

}

In this case, we do a complete override, but in some cases, you might want to use the behavior from the base class in addition to new functionality. This can be done by using the base keyword, which refers to the class we inherit from:

abstract class FourLeggedAnimal

{

public virtual string Describe()

{

return "This animal has four legs.";

}

class Dog : FourLeggedAnimal

{

public override string Describe()

{

string result = base.Describe();

result += " In fact, it's a dog!";

return result;

}

Now obviously, you can create other subclasses of the FourLeggedAnimal class - perhaps a cat or a lion? In the next chapter, we will do a more advanced example and introduce abstract methods as well. Read on.

More abstract classes

In the previous chapter, we had a look at abstract classes. In this chapter, we will expand the examples a bit, and throw in some abstract methods as well. Abstract methods are only allowed within abstract classes. Their definition will look like a regular method, but they have no code inside them:

abstract class FourLeggedAnimal

{

public abstract string Describe();

}

So, why would you want to define an empty method that does nothing? Because an abstract method is an obligation to implent that very method in all subclasses. In fact, it's checked at compile time, to ensure that your subclasses has this method defined. Once again, this is a great way to create a base class for something, while still maintaining a certain amount of control of what the subclasses should be able to do. With this in mind, you can always treat a subclass as its baseclass, whenever you need to use methods defined as abstract methods on the baseclass. For instance, consider the following example:

namespace AbstractClasses

{

class Program

{

static void Main(string[] args)

{

System.Collections.ArrayList animalList = new System.Collections.ArrayList();

animalList.Add(new Dog());

animalList.Add(new Cat());

foreach(FourLeggedAnimal animal in animalList)

Console.WriteLine(animal.Describe());

Console.ReadKey();

}

abstract class FourLeggedAnimal

{

public abstract string Describe();

}

class Dog : FourLeggedAnimal

{

public override string Describe()

{

return "I'm a dog!";

}

class Cat : FourLeggedAnimal

{

public override string Describe()

{

return "I'm a cat!";

}

As you can see, we create an ArrayList to contain our animals. We then instantiate a new dog and a new cat and add them to the list. They are instantiated as a Dog and a Cat respectively, but they are also of the type FourLeggedAnimal, and since the compiler knows that subclasses of that class contains the Describe() method, you are actually allowed to call that method, without knowing the exact type of animal. So by typecasting to the FourLeggedAnimal, which is what we do in the foreach loop, we get access to members of the subclasses. This can be very useful in lots of scenarios.

Interfaces

In previous chapters, we had a look at abstract classes. Interfaces are much like abstract classes and they share the fact that no instances of them can be created. However, interfaces are even more conceptual than abstract classes, since no method bodies are allowed at all. So an interface is kind of like an abstract class with nothing but abstract methods, and since there are no methods with actual code, there is no need for any fields. Properties are allowed though, as well as indexers and events. You can consider an interface as a contract - a class that implements it is required to implement all of the methods and properties. However, the most important difference is that while C# doesn't allow multiple inheritance, where classes inherit more than a single base class, it does in fact allow for implementation of multiple interfaces!

So, how does all of this look in code? Here's a pretty complete example. Have a look, perhaps try it out on your own, and then read on for the full explanation:

using System;

using System.Collections.Generic;

namespace Interfaces

{

class Program

{

static void Main(string[] args)

{

List<Dog> dogs = new List<Dog>();

dogs.Add(new Dog("Fido"));

dogs.Add(new Dog("Bob"));

dogs.Add(new Dog("Adam"));

dogs.Sort();

foreach(Dog dog in dogs)

Console.WriteLine(dog.Describe());

Console.ReadKey();

}

interface IAnimal

{

string Describe();

string Name

{

get;

set;

}

class Dog : IAnimal, IComparable

{

private string name;

public Dog(string name)

{

this.Name = name;

}

public string Describe()

{

return "Hello, I'm a dog and my name is " + this.Name;

}

public int CompareTo(object obj)

{

if(obj is IAnimal)

return this.Name.CompareTo((obj as IAnimal).Name);

return 0;

}

public string Name

{

get { return name; }

set { name = value; }

}

Let's start in the middle, where we declare the interface. As you can see, the only difference from a class declaration, is the keyword used - interface instead of class. Also, the name of the interface is prefixed with an I for Interface - this is simply a coding standard, and not a requirement. You can call your interfaces whatever you want, but since they are used like classes so much that you might have a hard time telling the difference in some parts of your code, the I prefix makes pretty good sense.

Then we declare the Describe method, and afterwards, the Name property, which has both a get and a set keyword, making this a read and writeable property. You will also notice the lack of access modifiers (public, private, protected etc.), and that's because they are not allowed in an interface - they are all public by default.

Next up is our Dog class. Notice how it looks just like inheriting from another class, with the colon between the class name and the class/interface being subclassed/implemented. However, in this case, two interfaces are implemented for the same class, simply separated by a comma. You can implement as many interfaces as you want to, but in this case we only implement two - our own IAnimal interface, and the .NET IComparable interface, which is a shared interface for classes that can be sorted. Now as you can see, we have implemented both the method and the property from the IAnimal interface, as well as a CompareTo method from the IComparable interface.

Now you might be thinking: If we have to do all the work our self, by implementing the entire methods and properties, why even bother? And a very good example of why it's worth your time, is given in the top of our example. Here, we add a bunch of Dog objects to a list, and then we sort the list. And how does the list know how to sort dogs? Because our Dog class has a CompareTo method that can tell how to compare two dogs. And how does the list know that our Dog object can do just that, and which method to call to get the dogs compared? Because we told it so, by implementing an interface that promises a CompareTo method! This is the real beauty of interfaces.

Debugging

Introduction to debugging

When you get past the most basic "Hello world!" examples, your code will reach a level of complexity where you can't necessarily figure out what's going on just by running it. What you need, is some black magic, which allows you to open the virtual hood of your application while it's running and see what's going on. Debugging is that magical tool, and as soon as you learn the most basic steps of it, you will wonder how you ever lived without it. It's a tool that every good programmer should understand, simply because it's almost impossible to fix bugs in complex code without it.

The most basic type of debugging, which is still being used by even advanced programmers, is sometimes called "print debugging" - a simple procedure, where you make your application print a piece of text or a number somewhere, allowing you to see which part of your code has been reached and what your variables contain. With C#, you can use the Console.Write() method, to output the contents of a variable or a simple status message, which will be printed to the console. That can be enough for some situations, but if you're using a nice IDE like Visual Studio or one of the Express versions, you have much stronger tools to your disposal, and they are every bit as easy to use, once you learn the most basic principles. In the next couple of chapters, we will guide you through the debugging possibilities of your IDE and after that, you will be a much stronger programmer.

Breakpoints

The very first thing about debugging that you need to know, is the breakpoint. It actually does exactly what the name implies - it marks a point in your code where the execution will take a break (and no, it won't actually break your code, don't worry). Placing a breakpoint in Visual Studio or one of the Express versions, is as simple as left-clicking in the gutter, which is the grey are to the left of your code. Once you click it, you will get a shiny, red circle as a reward - this circle marks where the debugger will halt when you execute your application. You better have a look for your self, and to see the effect, we will use the following piece of code:

namespace DebugTest

{

class Program

{

static void Main(string[] args)

{

int a = 5, b = 8, c = 233;

int d = a + c - b;

Console.WriteLine(d);

}

Now, can you predict the result just from looking at the code? Probably, and if not, you could just get out the old calculator and do the math, but that's not really the point. Just imagine the amount of code being much bigger, and let's debug the thing! Place a breakpoint by clicking in the left gutter - your IDE should now look something like this:

Description: Breakpoint

Okay, you're ready to start your first debugging session. As soon as you have placed the breakpoint, you can just run your application like you normally would - from the menu, the toolbar or by pressing F5. What happens now is that the application is executed just like normal, but as soon as a line with a breakpoint is reached, the execution is stopped right before that line would be executed. In this case, it means that the variables a, b and c will have a value, but d will only have it's default value (which is 0 for an integer), since it won't be set before the line with the breakpoint has been evaluated. Now, here comes the cool part - try hovering your mouse over the different variables - the IDE will tell you what they contain. As mentioned, the d variable will have it's default value, but let's change that, by moving forward in the execution. In the next chapter, I will show you how to navigate around your code, while it's being executed.

Stepping through the code

In this chapter, we will look into stepping through your code, which is another very essential part of debugging. For the purpose, I have written this simple application:

namespace DebugTest

{

class Program

{

static void Main(string[] args)

{

int a = 5;

a = a * 2;

a = a - 3;

a = a * 6;

Console.WriteLine(a);

}

It simply manipulates the variable "a" a couple of times and the outputs the final result. Try placing a breakpoint, as described in the previous chapter, on the very first line where a is used (and declared). Now run the application. The execution is stopped and you can hover your mouse over the a, to ensure that what we learned in the previous chapter is in fact true: The variable only contains the default value, because the code that assigns the value (in this case 5), has not been executed yet, but let's change that. From the Debug menu, select the "Step over" option, or even better, use the keyboard shortcut F10. The execution will now proceed to the next relevant line and if you hover your mouse over the a variable, you will now see that it has a value. Try again, and you will see the value change according to the lines being executed one by one, until you have reached the end.

Okay, so that was pretty basic, but also very useful, as you will realise once you start writing more complicated code. In this example, the flow of the code was very simple, since we stayed within a single function, but what if your code starts spreading over multiple classes and/or functions? Try this example:

namespace DebugTest

{

class Program

{

static void Main(string[] args)

{

int a = 5;

int b = 2;

int result = MakeComplicatedCalculation(a, b);

Console.WriteLine(result);

}

static int MakeComplicatedCalculation(int a, int b)

{

return a * b;

}

Place a breakpoint on the first line of the Main method and run the application. Now use the "Step over" function to step through each line. As you will see, it moves over the function call without any notice - that's simply how debugging works. Now, try again, from the start, and once you have stepped to the line with the MakeComplicatedCalculation() call, select Debug -> Step into, or use the keyboard shortcut F11. The debugger now steps into the first possible function call, allowing you to step through that function as well. As you can probably imagine, this allows you to step through a complicated block of code, while only entering the function calls that interests you.

If you step into a function and then realise that you would rather return to the previous context, you use the very logically named option from the Debug menu called "Step out" (keyboard shortcut is Shift+F11). This will return you to your previous context, which obviously means that you can step into as many function calls as you want to, and then find your way back by using the step out option.

The tool windows

When debugging in Visual Studio or one of the Express versions, the tool windows in the bottom of the screen will change and new windows will be revealed (unless you have turned them off). The windows are called something along the lines of "Locals", "Watch", "Call stack" and "Immediate window" and they are all related to the debugging experience. In this chapter we will look into each of them and show you what they can do for you.

Locals

This window is the most simple of them all. When a breakpoint is hit, all local variables will be listed here, allowing you to get a quick overview of their name, type and value. You can even right-click in the grid and select "Edit value", to give a variable a new value. This allows you to test your code under other conditions than the current ones.

Watch

The Watch window is a bit like the Locals window, only here you get to decide which variables are tracked, local or global. You can add variables to watch over by dragging them from the code window, from the Locals window or by writing its name on the last, empty line. Your variables will stay in the Watch window until you remove it again, but will only be updated when you are debugging within the current scope. For instance, a variable in function A will not be updated when you are stepping through function B. Just like with the Locals window, you can right-click a watched variable and select "Edit value" to change the current value of the variable.

Call Stack

The Call Stack window will show you the current hierarchy of called functions. For instance, if function A calls function B which calls function C which then calls function D, the Call Stack window will show it, and you will be able to jump to each of the function declarations. You can also see which parameters were passed to each function. In the simple examples we have worked with so far, this might seem pointless, since keeping track of which function calls which function is trivial, but as soon as your code reaches a higher level of complexity and you have function in classes calling function in other classes, the Call Stack can be a real life saver.

Immediate window

The Immediate window is probably the most useful of them all. It allows you to execute custom lines of code in the current context of the debugger. This allows you to check variables, alter their values or just simply test a line of code. You simply type it into the window, hit Enter and the line will be executed. Type a variable name, and its value will be printed. Set a variable value by writing a = 5. The result, if any, will be printed and any changes you make, will be reflected when you continue the execution of the code. The Immediate window is like a C# terminal, where you can enter code and see the results immediately - once you get used to it, you might become addicted. I know I am.

Advanced breakpoints

In a previous chapter, we set the first breakpoint and it was good. However, there is actually more to breakpoints than that, at least if you're using Visual Studio. Unfortunately, it seems that Microsoft has disabled these extra debugging features in some of their Express versions, but don't worry: They are certainly nice to have, but you can get by without them. However, for those with access to Visual Studio, here is the most interesting breakpoint related features. You get access to them by setting a breakpoint, right-clicking it with the mouse and then selecting the desired function.

Condition

This option allows you to specify a condition that has to be true or changed, for the breakpoint to be hit. This can be really useful when dealing with more advanced code, where you want the execution to stop only under certain circumstances. For instance, you might have a loop that iterates a bunch of time before the relevant code is reached - in a situation like that, you could simply place a breakpoint and then configure an appropriate condition. Here is a rather boring example, which will show you how it works:

static void Main(string[] args)

    for(int i = 0; i < 10; i++)

        Console.WriteLine("i is " + i);

Set a breakpoint on the line where we do output to the console. Now run the application - the breakpoint is triggered each time the loop iterates. But perhaps that's not what we want. Maybe we only want it to be hit when i equals 4 (the 5th iteration). Do that by defining a simple condition like this:

i == 4

The breakpoint will now get a little, white plus inside it and when you run the application, it will only break when the i variable equals 4. You can also use the "has changed" option to instruct the debugger to only halt the execution if the result of the above statement has changed, for instance from false to true.

Hit count

With this dialog, you can define an alternative condition, based on the amount of times the breakpoint has been hit. For instance, you can decide that your breakpoint should not halt the execution until it has been hit a certain amount of times. There are various options to control this, depending on what you need, and during debug time, you can check this dialog to see how many times the breakpoint has been hit so far.

When hit...

Using this dialog, you can define alternative behaviour for when your breakpoint is hit. This can come in handy in lots of situations, where you don't want the execution to stop, but simply get a status message printed or a macro activated. It allows you to define a custom message which will be printed, where you can include all kinds of information about the execution. For advanced users, the option to get a specific macro executed when a breakpoint is hit will also be useful.

Advanced topics

Enumerations

Enumerations are special sets of named values which all maps to a set of numbers, usually integers. They come in handy when you wish to be able to choose between a set of constant values, and with each possible value relating to a number, they can be used in a wide range of situations. As you will see in our example, enumerations are defined above classes, inside our namespace. This means we can use enumerations from all classes within the same namespace.

Here is an example of a simple enumeration to show what they are all about.

public enum Days { Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday }

All of these possible values correspond to a number. If we don't set them specifically, the first value is equal to 0, the next one to 1, and so on. The following piece of code will prove this, as well as show how we use one of the possible values from the enum:

using System;

namespace ConsoleApplication1

{

public enum Days { Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday }

class Program

{

static void Main(string[] args)

{

Days day = Days.Monday;

Console.WriteLine((int)day);

Console.ReadLine();

}

The output will be zero, because the Monday value maps directly to the number zero. Obviously we can change that - change the line to something like this:

public enum Days { Monday = 1, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday }

If you run our code again, you will see that the Monday now equals 1 instead of 0. All of the other values will be one number higher as well as a result. You can assign other numbers to the other values as well. Because of the direct mapping to a number, you can use numbers to get a corresponding value from the enumeration as well, like this:

Days day = (Days)5;

Console.WriteLine(day);

Console.ReadLine();

Another cool feature of enumerations is the fact that you can a string representation of the values as well. Change the above example to something like this:

static void Main(string[] args)

{

string[] values = Enum.GetNames(typeof(Days));

foreach(string s in values)

Console.WriteLine(s);

Console.ReadLine();

}

The Enum class contains a bunch of useful methods for working with enumerations.

Exception handling

In every program, things go wrong sometimes. With C#, we're blessed with a good compiler, which will help us prevent some of the most common mistakes. Obviously it can't see every error that might happen, and in those cases, the .NET framework will throw an exception, to tell us that something went wrong. In an earlier chapter, about arrays, I described how we would get an exception if we tried to stuff too many items into an array. Let's bring the example:

using System;

using System.Collections;

namespace ConsoleApplication1

{

class Program

{

static void Main(string[] args)

{

int[] numbers = new int[2];

numbers[0] = 23;

numbers[1] = 32;

numbers[2] = 42;

foreach(int i in numbers)

Console.WriteLine(i);

Console.ReadLine();

}

Okay, try running this example, and you will see what I'm talking about. Do you see what we're doing wrong? We have defined an array of integers with room for 2 items, yet we try to use 3 spaces in it. Obviously, this leads to an error, which you will see if you try to run this example. When run inside Visual C# Express, the IDE gives us some options for the exception, but if you try to execute the program by simply doubleclicking the EXE file, you will get a nasty error. If you know that an error might occur, you should handle it. This is where exceptions are used. Here is a slightly modified version of the code from above:

int[] numbers = new int[2];

try

{

numbers[0] = 23;

numbers[1] = 32;

numbers[2] = 42;

foreach(int i in numbers)

Console.WriteLine(i);

}

catch

{

Console.WriteLine("Something went wrong!");

}

Console.ReadLine();

Let me introduce to you your new best friend when it comes to error handling: the try..catch block. Try running the program now, and see the difference - instead of Visual C# Express/Windows telling us that a serious problem occured, we get to tell our own story. But wouldn't it be nice if we could tell what went wrong? No problem:

catch(Exception ex)

{

Console.WriteLine("An error occured: " + ex.Message);

}

As you can see, we have added something to the catch statement. We now tell which exception we want caught, in this case the base of all exceptions, the Exception. By doing so, we get some information about the problem which caused the exception, and by outputting the Message property, we get an understandable description of the problem.

As I said, Exception is the most general type of exception. The rules of exception handling tells us that we should always use the least general type of exception, and in this case, we actually know the exact type of exception generated by our code. How? Because Visual Studio told us when we didn't handle it. If you're in doubt, the documentation usually describes which exception(s) a method may throw. Another way of finding out is using the Exception class to tell us - change the output line to this:

Console.WriteLine("An error occured: " + ex.GetType().ToString());

The result is, as expected, IndexOutOfRangeException. We should therefore handle this exception, but nothing prevents us from handling more than one exception. In some situations you might wish to do different things, depending on which exception was thrown. Simply change our catch block to the following:

catch(IndexOutOfRangeException ex)

{

Console.WriteLine("An index was out of range!");

}

catch(Exception ex)

{

Console.WriteLine("Some sort of error occured: " + ex.Message);

}

As you can see, we look for the IndexOutOfRangeException first. If we did it the other way around, the catch block with the Exception class would get it, because all exceptions derive from it. So in other words, you should use the most specific exceptions first.

One more thing you should know about concerning exceptions is the finally block. The finally block can be added to a set of catch blocks, or be used exclusively, depending on your needs. The code within the finally block is always run - exception or no exception. It's a good place if you need to close file references or dispose objects you won't need anymore. Since our examples have been pretty simple so far, we haven't really been in need of any cleanup, since the garbage collector handles that. But since will likely run into situations where you need the finally block, here is an extended version of our example:

int[] numbers = new int[2];

try

{

numbers[0] = 23;

numbers[1] = 32;

numbers[2] = 42;

foreach(int i in numbers)

Console.WriteLine(i);

}

catch(IndexOutOfRangeException ex)

{

Console.WriteLine("An index was out of range!");

}

catch(Exception ex)

{

Console.WriteLine("Some sort of error occured: " + ex.Message);

}

finally

{

Console.WriteLine("It's the end of our try block. Time to clean up!");

}

Console.ReadLine();

If you run the code, you will see that both the first exception block and the finally block is executed. If you remove the line that adds the number 42 to the array, you will see that only the finally block is reached.

Another important part you should know about exceptions, is how they impact the method in which the exceptions occur. Not all unhandled exceptions are fatal for your application, but when they aren't, you should not expect the remaining code of the method to be executed. On the other hand, if you do handle the exception, only the lines after the try block will be executed. In our example, the loop that outputs the values of the array is never reached, because the try block goes straight to the catch/finally block(s) once an exception is thrown. However, the last line, where we read from the console to prevent the application from exiting immediately, is reached. You should always have this in mind when you construct try blocks.

Structs

The C# struct is a lightweight alternative to a class. It can do almost the same as a class, but it's less "expensive" to use a struct rather than a class. The reason for this is a bit technical, but to sum up, new instances of a class is placed on the heap, where newly instantiated structs are placed on the stack. Furthermore, you are not dealing with references to structs, like with classes, but instead you are working directly with the struct instance. This also means that when you pass a struct to a function, it is by value, instead of as a reference. There is more about this in the chapter about function parameters.

So, you should use structs when you wish to represent more simple data structures, and especially if you know that you will be instantiating lots of them. There are lots of examples in the .NET framework, where Microsoft has used structs instead of classes, for instance the Point, Rectangle and Color struct.

First I would like to show you an example of using a struct, and then we will discuss some of the limitations of using them instead of classes:

class Program

{

static void Main(string[] args)

{

Car car;

car = new Car("Blue");

Console.WriteLine(car.Describe());

car = new Car("Red");

Console.WriteLine(car.Describe());

Console.ReadKey();

}

struct Car

{

private string color;

public Car(string color)

{

this.color = color;

}

public string Describe()

{

return "This car is " + Color;

}

public string Color

{

get { return color; }

set { color = value; }

}

The observant reader will notice that this is the exact same example code as used in the introduction to classes, besides the change from a class to a struct. This goes to show how similar the two concepts are. But how do they differ, besides the technical details mention in the beginning of this chapter?

First of all, fields can't have initializers, meaning that you can't declare a member like this:

private string color = "Blue";

If you declare a constructor, all fields must be assigned to before leaving the constructor. A struct does come with a default constructor, but as soon as you choose to define your own, you agree to initialize all fields in it. That also means that you can't declare your own paramaterless constructor - all struct constructors has to take at least one parameter. In our example above, we did in fact assign a value to the color field. If we hadn't done that, the compiler would complain.

A struct can not inherit from other classes or structs, and classes can't inherit from structs. A struct does inherit from the Object class, but that's it for inheritance and structs. They do support interfaces though, meaning that your structs can implement custom interfaces.

C# 3.0

Introduction to C# 3.0

With the release of Microsoft .NET framework 3.5 and Visual Studio 2008, codenamed "Orcas", a bunch of new features were added to the C# language, under the name C# version 3. In the following chapters, I will show you some of the cool, new stuff that Microsoft added, in a constant effort to make it easier and faster for all their programmers to write good code.

Please be aware that you will need at least version 3.5 of the framework installed, as well as a version 2008 of your favourite IDE, either Visual Studio or one of the Express versions, to compile and take advantage of the examples.

Automatic properties

A real pain in the neck for all programmers writing object oriented code has always been declaring public properties for all the private fields. This is a lot of tedious work, especially because almost all properties will be a simple get and set mapping to the private field, without anything clever added, like this:

private string name;

public string Name

{

get { return name; }

set { name = value; }

}

With a simple property like that, we could pretty much just as well have declared the field as public and used it directly, instead of adding the extra layer of a property. However, the guidelines of OOP tells us to do it this way, and most of us resists the temptation of doing it the easy way. With C# 3.0 we don't have to deal with this dilemma anymore! The above example can now be written like this instead:

public string Name

{

get;

set;

}

Or using even less space, like this:

public string Name { get; set; }

No field declaration, and no code to get and set the value of the field. All of that is handled automatically by the compiler, which will automatically create a private field and populate the get and set method with the basic code required to read and write the field. From the outside, this will look just like a regular property, but you saved a lot of extra keystrokes and your class will be less bloated. Of course, you can still use the old way, as shown in our first example - this is simply an extra feature that you can use, if you feel like it.

Object Initializers

With C# 3.0, initializing both objects and collections have become much easier. Consider this simple Car class, where we use the automatic properties described in a previous chapter:

class Car

{

public string Name { get; set; }

public Color Color { get; set; }

}

Now, in C# 2.0, we would have to write a piece of code like this to create a Car instance and set its properties:

Car car = new Car();

car.Name = "Chevrolet Corvette";

car.Color = Color.Yellow;

It's just fine really, but with C# 3.0, it can be done a bit more cleanly, thanks to the new object initializer syntax:

Car car = new Car { Name = "Chevrolet Corvette", Color = Color.Yellow };

As you can see, we use a set of curly brackets after instantiating a new Car object, and within them, we have access to all the public properties of the Car class. This saves a bit of typing, and a bit of space as well. The cool part is that it can be nested too. Consider the following example, where we add a new complex property to the Car class, like this:

class Car

{

public string Name { get; set; }

public Color Color { get; set; }

public CarManufacturer Manufacturer { get; set; }

}

class CarManufacturer

{

public string Name { get; set; }

public string Country { get; set; }

}

To initialize a new car with C# 2.0, we would have to do something like this:

Car car = new Car();

car.Name = "Corvette";

car.Color = Color.Yellow;

car.Manufacturer = new CarManufacturer();

car.Manufacturer.Name = "Chevrolet";

car.Manufacturer.Country = "USA";

With C# 3.0, we can do it like this instead:

Car car = new Car {

Name = "Chevrolet Corvette",

Color = Color.Yellow,

Manufacturer = new CarManufacturer {

Name = "Chevrolet",

Country = "USA"

}

};

Or in case you're not too worried about readability, like this:

Car car = new Car { Name = "Chevrolet Corvette", Color = Color.Yellow, Manufacturer = new CarManufacturer { Name = "Chevrolet", Country = "USA" } };

Just like with the automatic properties, this is syntactical sugar - you can either use it, or just stick with the old, fashioned way of doing things.

Extension Methods

Another cool feature of C# 3.0 is Extension Methods. They allow you to extend an existing type with new functionality, without having to sub-class or recompile the old type. For instance, you might like to know whether a certain string was a number or not. The usual approach would be to define a function and then call it each time, and once you got a whole lot of those kind of functions, you would put them together in a utility class, like this:

public class MyUtils

{

public static bool IsNumeric(string s)

{

float output;

return float.TryParse(s, out output);

}

Now you could check a string by executing a line of code like this:

string test = "4";

if (MyUtils.IsNumeric(test))

Console.WriteLine("Yes");

else

Console.WriteLine("No");

However, with Extension Methods, you can actually extend the String class to support this directly. You do it by defining a static class, with a set of static methods that will be your library of extension methods. Here is an example:

public static class MyExtensionMethods

{

public static bool IsNumeric(this string s)

{

float output;

return float.TryParse(s, out output);

}

The only thing that separates this from any other static method, is the "this" keyword in the parameter section of the method. It tells the compiler that this is an extension method for the string class, and that's actually all you need to create an extension method. Now, you can call the IsNumeric() method directly on strings, like this:

string test = "4";

if (test.IsNumeric())

Console.WriteLine("Yes");

else

Console.WriteLine("No");

File Handling

Reading and writing files

In this chapter, we will look into reading and writing simple files with C#. Fortunately for us, C# makes it very easy. The File class, from the Syste.IO namespace comes with pretty much everything we could possibly want, making it very easy to do simple reading and writing of a file.

In our first example, we will construct an extremely minimalistic text editor. In fact, it is so simple that we can only read one file and then write new content to it, and only a single line of text at a time. But it shows just how easy it is to use the File class:

using System;

using System.IO;

namespace FileHandlingArticleApp

{

class Program

{

static void Main(string[] args)

{

if(File.Exists("test.txt"))

{

string content = File.ReadAllText("test.txt");

Console.WriteLine("Current content of file:");

Console.WriteLine(content);

}

Console.WriteLine("Please enter new content for the file:");

string newContent = Console.ReadLine();

File.WriteAllText("test.txt", newContent);

}

You will notice that we use the File class in three places: We use it to check if our file exists, we use the ReadAllText() method to read the content of the file, and we use the WriteAllText() method to write new content to the file. You will notice that I'm not using absolute paths, but just a simple filename. This will place the file in the same directory as the executable file, which is fine for now. Other than that, the example should be easy enough to understand: We check for the file, if it exists, we read its content and output it to the console. Then we prompt the user for new content, and once we have it, we write it to the file. Obviously that will overwrite the previous content, but for now, that's just fine. We could however use the AppendAllText method instead. Try changing the WriteAllText line to this instead:

File.AppendAllText("test.txt", newContent);

If you run it, you will see that the new text is added to the existing text instead of overwriting it. As simple as that. But we still get only one line of text per execution of our application. Let's be a bit creative and change that. Replace the last lines in our example with this:

Console.WriteLine("Please enter new content for the file - type exit and press enter to finish editing:");

string newContent = Console.ReadLine();

while(newContent != "exit")

{

File.AppendAllText("test.txt", newContent + Environment.NewLine);

newContent = Console.ReadLine();

}

As you can see, we instruct the user to enter the word exit when they wish to stop editing the file, and until they do just that, we append the user input to the file and then prompt for a new line. We also append a newline character, the Environment.NewLine, to make it look like actual lines of text. However, instead of writing to the file each time, a more pretty solution would probably look something like this:

Console.WriteLine("Please enter new content for the file - type exit and press enter to finish editing:");

using(StreamWriter sw = new StreamWriter("test.txt"))

{

string newContent = Console.ReadLine();

while(newContent != "exit")

{

sw.Write(newContent + Environment.NewLine);

newContent = Console.ReadLine();

}

The usage of Streams is a bit out of the scope of this chapter, but the cool thing about it in this example is that we only open the file once, and then write the changes to it once we're satisfied. In this case, we're taking advantage of the using() statement of C#, which ensures that the file reference is closed once it goes out of scope, which is when it's block of { } is done. If you don't use the using() statement, you will have to manually call the Close() method on the StreamWriter instance.

Manipulating files and directories

In the previous chapter, we looked into reading and writing text with simple files. We used the File class for this, but it does a lot more than just reading and writing. When combined with the Directory class, we can perform pretty much any filesystem operation, like renaming, moving, deleting and so on.

This chapter will provide numerous examples of doing just those things. The explanations will be limited, since the methods used are pretty simple and easy to use. You should be aware of two things: First of all, make sure that you import the System.IO namespace like this:

using System.IO;

Also, be aware that we are not doing any exception handling here. We will check that files and directories exist before using it, but there's no exception handling, so in case anything goes wrong, the application will crash. Exception handling is generally a good idea when doing IO operations. For more information, please read the exception handling chapter in this tutorial.

In all of the examples, we use simple file and directory names, which will have to exist in the same directory as the generated EXE file of your application. In the project settings you can see where your EXE file is generated to.

Collection Initializers

Just like C# 3.0 offers a new way of initializing objects, a new syntax for initializing a list with a specific set of items added to it, has been included. We can use the Car class from the last chapter:

class Car

{

public string Name { get; set; }

public Color Color { get; set; }

}

If we wanted to create a list to contain a range of cars, we would have to do something like this with C# 2.0:

Car car;

List<Car> cars = new List<Car>();

car = new Car();

car.Name = "Corvette";

car.Color = Color.Yellow;

cars.Add(car);

car = new Car();

car.Name = "Golf";

car.Color = Color.Blue;

cars.Add(car);

Using object initializers, we could do it a bit shorter:

List<Car> cars = new List<Car>();

cars.Add(new Car { Name = "Corvette", Color = Color.Yellow });

cars.Add(new Car { Name = "Golf", Color = Color.Blue});

However, it can be even simpler, when combined with collection initializers:

List<Car> cars = new List<Car>

{

new Car { Name = "Corvette", Color = Color.Yellow },

new Car { Name = "Golf", Color = Color.Blue}

};

Or in the one-line version, which does exactly the same:

List<Car> cars = new List<Car> { new Car { Name = "Corvette", Color = Color.Yellow }, new Car { Name = "Golf", Color = Color.Blue} };

10 lines of code has been reduced to a single, albeit a bit long, line, thanks to object and collection initializers.

Deleting a file

if(File.Exists("test.txt"))

    File.Delete("test.txt");

    if(File.Exists("test.txt") == false)

        Console.WriteLine("File deleted...");

else

    Console.WriteLine("File test.txt does not yet exist!");

Console.ReadKey();

Deleting a directory

if(Directory.Exists("testdir"))

    Directory.Delete("testdir");

    if(Directory.Exists("testdir") == false)

        Console.WriteLine("Directory deleted...");

else

    Console.WriteLine("Directory testdir does not yet exist!");

Console.ReadKey();

If testdir is not empty, you will receive an exception. Why? Because this version of Delete() on the Directory class only works on empty directories. It's very easy to change though:

Directory.Delete("testdir", true);

The extra parameter makes sure that the Delete() method is recursive, meaning that it will traverse subdirectories and delete the files of it, before deleting the directories.

Renaming a file

if(File.Exists("test.txt"))

    Console.WriteLine("Please enter a new name for this file:");

    string newFilename = Console.ReadLine();

    if(newFilename != String.Empty)

        File.Move("test.txt", newFilename);

        if(File.Exists(newFilename))

            Console.WriteLine("The file was renamed to " + newFilename);

            Console.ReadKey();

You will notice that we use the Move() method to rename the file. Why not a Rename() method? Because there are no such method, since moving and renaming is essentially the same thing.

Renaming a directory

if(Directory.Exists("testdir"))

    Console.WriteLine("Please enter a new name for this directory:");

    string newDirName = Console.ReadLine();

    if(newDirName != String.Empty)

        Directory.Move("testdir", newDirName);

        if(Directory.Exists(newDirName))

            Console.WriteLine("The directory was renamed to " + newDirName);

            Console.ReadKey();

Creating a new directory

Console.WriteLine("Please enter a name for the new directory:");

string newDirName = Console.ReadLine();

if(newDirName != String.Empty)

    Directory.CreateDirectory(newDirName);

    if(Directory.Exists(newDirName))

        Console.WriteLine("The directory was created!");

        Console.ReadKey();

File and directory information

The File and the Directory classes, which we have used in the previous couple of chapters, are great for direct file and directory manipulation. However, sometimes we wish to get information on them instead, and once again, the System.IO namespace comes to our rescue: The FileInfo and DirectoryInfo classes. In this chapter, we will look into some of the ways to use these two classes.

First, let's explore a simple way to use the FileInfo class.

static void Main(string[] args)

{

FileInfo fi = new FileInfo(System.Reflection.Assembly.GetExecutingAssembly().Location);

if(fi != null)

Console.WriteLine(String.Format("Information about file: {0}, {1} bytes, last modified on {2} - Full path: {3}", fi.Name, fi.Length, fi.LastWriteTime, fi.FullName));

Console.ReadKey();

}

We create a new instance of the FileInfo class. It takes one parameter, which is the path to the file we want information about. We could have just specified a filename, but for fun, I thought it would be cool to get information about the actual application we are working on, that is, the EXE file that our project is compiled into. Since we don't have access to the Application project in a Console application (it's part of the WinForms assembly), we use a bit of Reflection to get the path to the current assembly. This is all way out of the scope of this particular chapter, but at least now you know.

Once we have a FileInfo instance, we output all sorts of information about it. Try running the application and you will see. All very nice and easy, and if you look at the FileInfo class, you will see that it offers even more information, as well as shortcuts to the methods found on the File class - and why not? We have a reference to the file anyway with the FileInfo instance, so we might as well get the same options as on the File class.

Now, information about a single file is just fine, but using the DirectoryInfo class, we can get information about all files and directories within a directory, which is obviously a very common scenario. Let me show you with a simple example:

DirectoryInfo di = new DirectoryInfo(Path.GetDirectoryName(System.Reflection.Assembly.GetExecutingAssembly().Location));

if(di != null)

{

FileInfo[] subFiles = di.GetFiles();

if(subFiles.Length > 0)

{

Console.WriteLine("Files:");

foreach(FileInfo subFile in subFiles)

{

Console.WriteLine(" " + subFile.Name + " (" + subFile.Length + " bytes)");

}

Console.ReadKey();

}

Instead of a FileInfo instance, we create a DirectoryInfo instance. We use the same trick to get the path of the executing file, and then the GetDirectoryName() method from the Path class, to get the directory part of the path only. We use the GetFiles() method to get an array of FileInfo instances, each representing a file in the directory. We then loop through it, printing out each filename and size.

Perhaps we want the directories as well. It's just as easy:

DirectoryInfo[] subDirs = di.GetDirectories();

if(subDirs.Length > 0)

{

Console.WriteLine("Directories:");

foreach(DirectoryInfo subDir in subDirs)

{

Console.WriteLine(" " + subDir.Name);

}

In some situations, you might only want files or directories with a specific name or file extension. Fortunately, FileInfo and DirectoryInfo has some pretty good support for that as well.

This will give us all files in the directory with a .exe extension:

FileInfo[] subFiles = di.GetFiles("*.exe");

This will give us all the directories which have the word "test" somewhere in the name:

DirectoryInfo[] subDirs = di.GetDirectories("*test*");

We can even find both files and directories recursively, which means that it will search in subdirectories of subdirectories of.... the originial directory:

FileInfo[] subFiles = di.GetFiles("*.exe", SearchOption.AllDirectories);

To only search the toplevel directory, the code would have to look like this:

FileInfo[] subFiles = di.GetFiles("*.exe", SearchOption.TopDirectoryOnly);

Reflection

Reflection introduction

Wikipedia says that "In computer science, reflection is the process by which a computer program can observe and modify its own structure and behaviour". This is exactly how Reflection in C# works, and while you may not realize it at this point, being able to examine and change information about your application during runtime, offers huge potential. Reflection, which is both a general term, as well as the actual name of the reflection capabilities in C#, works very, very well, and it's actually not that hard to use. In the next couple of chapters, we will go more into depth about how it works and provide you with some cool examples, which should show you just how useful Reflection is.

However, to get you started and hopefully interested, here is a small example. It solves a question that I have seen from many newcomers to any programming language: How can I change the value of a variable during runtime, only by knowing its name? Have a look at this small demo application for a solution, and read the next chapters for an explanation of the different techniques used.

using System;

using System.Collections.Generic;

using System.Text;

using System.Reflection;

namespace ReflectionTest

{

class Program

{

private static int a = 5, b = 10, c = 20;

static void Main(string[] args)

{

Console.WriteLine("a + b + c = " + (a + b + c));

Console.WriteLine("Please enter the name of the variable that you wish to change:");

string varName = Console.ReadLine();

Type t = typeof(Program);

FieldInfo fieldInfo = t.GetField(varName, BindingFlags.NonPublic | BindingFlags.Static);

if(fieldInfo != null)

{

Console.WriteLine("The current value of " + fieldInfo.Name + " is " + fieldInfo.GetValue(null) + ". You may enter a new value now:");

string newValue = Console.ReadLine();

int newInt;

if(int.TryParse(newValue, out newInt))

{

fieldInfo.SetValue(null, newInt);

Console.WriteLine("a + b + c = " + (a + b + c));

}

Console.ReadKey();

}

Try running the code and see how it works. Besides the lines where we use the actual Reflection, it's all very simple. Now, go to the next chapter for some more theory on how it works.

The right Type

The Type class is the foundation of Reflection. It serves as runtime information about an assembly, a module or a type. Fortunately, obtaining a reference to the Type of an object is very simply, since every class that inherits from the Object class, has a GetType() method. If you need information about a non-instantiated type, you may use the globally available typeof() method, which will do just that. Consider the following examples, where we use both approaches:

using System;

using System.Collections.Generic;

using System.Text;

using System.Reflection;

namespace ReflectionTest

{

class Program

{

static void Main(string[] args)

{

string test = "test";

Console.WriteLine(test.GetType().FullName);

Console.WriteLine(typeof(Int32).FullName);

Console.ReadKey();

}

We use the GetType() method on our own variable, and then we use the typeof() on a known class, the Int32. As you can see, the result in both cases is a Type object, for which we can read the FullName property.

At some point, you might even only have the name of the type that you're looking for. In that case, you will have to get a reference to it from the proper assembly. In the next example, we get a reference to the executing assembly, that is, the assembly from where the current code is being executed from, and then we list all of it's types:

using System;

using System.Collections.Generic;

using System.Text;

using System.Reflection;

namespace ReflectionTest

{

class Program

{

static void Main(string[] args)

{

Assembly assembly = Assembly.GetExecutingAssembly();

Type[] assemblyTypes = assembly.GetTypes();

foreach(Type t in assemblyTypes)

Console.WriteLine(t.Name);

Console.ReadKey();

}

class DummyClass

{

//Just here to make the output a tad less boring :)

}

The output will be the name of the two declared classes, Program and DummyClass, but in a more complex application, the list would probably be more interesting. In this case, we only get the name of the type, but obviously we would be able to do a lot more, with the Type reference that we get. In the next chapters, I will show you a bit more on what we can do with it.

Instantiating a class

So far, we have worked with .NET types or objects already instantiated. But with Reflection, we can actually do the instantiation at runtime as well, knowing the name of the class we wish to instantiate. There are several ways of doing it, but I prefer getting a reference to the constructor that I wish to use, invoke it, and then use the returned value as my instance. Here's an example of doing just that. Code first, then I will explain it all:

using System;

using System.Collections.Generic;

using System.Text;

using System.Reflection;

namespace ReflectionTest

{

class Program

{

static void Main(string[] args)

{

Type testType = typeof(TestClass);

ConstructorInfo ctor = testType.GetConstructor(System.Type.EmptyTypes);

if(ctor != null)

{

object instance = ctor.Invoke(null);

MethodInfo methodInfo = testType.GetMethod("TestMethod");

Console.WriteLine(methodInfo.Invoke(instance, new object[] { 10 }));

}

Console.ReadKey();

}

public class TestClass

{

private int testValue = 42;

public int TestMethod(int numberToAdd)

{

return this.testValue + numberToAdd;

}

I have defined a simple class for testing this, called TestClass. It simply contains a private field and a public method. The method returns the value of the private property, with the value of the parameter added to it. Now, what we want is to create a new instance of this TestClass, call the TestMethod and output the result to the console.

In this example, we have the luxury of being able to use the typeof() directly on the TestClass, but at some point, you may have to do it solely by using the name of the desired class. In that case, you can get a reference to it through the assembly where it is declared, as demonstrated in the chapter about Type.

So, with a Type reference to the class, we ask for the default constructor by using the GetConstructor() method, passing System.Type.EmptyTypes as a parameter. In case we wanted a specific constructor, we would have to provide an array of Type's, each defining which parameter the constructor we were looking for, would take.

Once we have a reference to the constructor, we simply call the Invoke() method to create a new instance of the TestClass class. We pass null as the parameter to Invoke(), since we're not looking to specify any parameters. We use the GetMethod(), along with the name of the method we want, to get the TestMethod() function, and then we once again use the magic of the Invoke() method to call this function. This time we need to specify a parameter, in the form of an array of objects. We do that on-the-fly, specifying the number 10 as the only parameter we need, and we then output the result of the method invocation. All of this through the magic of Reflection!

A Reflection based settings class

Okay, so I thought that I would end this part of the tutorial about Reflection, with a cool and useful example. It's a bit bigger than the usual examples here at the site, but hopefully you will find it really useful. It uses a bunch of the stuff that we have looked into during the last couple of chapters, so hopefully you can keep up.

A common scenario when creating any sort of application, is the desire to save the users settings. When you get several settings, you will probably create a Settings class, which will handle loading and saving of the desired settings. Each time you need a new setting in your Settings class, you will have to update the Load() and Save() methods, to include this new setting. But hey, why not let the Settings class discover its own properties and then load and save them automatically? With Reflection, it's quite easy, and if you have read the other chapters in the Reflection section of this tutorial, you should be able to understand the following example.

To make it fit better into a small example, I am saving information about a person instead of application settings, but hopefully you will get the general idea anyway. Please be aware that using Reflection WILL be slower than reading and writing known properties manually, so you should consider when to use it and when to opt for a faster approach! Also, in our example, we use a simple text file for storing even simpler values, only separated by a | (pipe character). If you're using this for real world stuff, you will probably want a better format for your data, perhaps XML. And of course, there is not much error handling, so you should probably add some of that as well.

Okay, let's get started. First, our Person class, which you can simply rename to Settings or something like that, to make it more useful to you:

public class Person

{

private int age = -1;

private string name = String.Empty;

public void Load()

{

if(File.Exists("settings.dat"))

{

Type type = this.GetType();

string propertyName, value;

string[] temp;

char[] splitChars = new char[] { '|' };

PropertyInfo propertyInfo;

string[] settings = File.ReadAllLines("settings.dat");

foreach(string s in settings)

{

temp = s.Split(splitChars);

if(temp.Length == 2)

{

propertyName = temp[0];

value = temp[1];

propertyInfo = type.GetProperty(propertyName);

if(propertyInfo != null)

this.SetProperty(propertyInfo, value);

}

public void Save()

{

Type type = this.GetType();

PropertyInfo[] properties = type.GetProperties();

TextWriter tw = new StreamWriter("settings.dat");

foreach(PropertyInfo propertyInfo in properties)

{

tw.WriteLine(propertyInfo.Name + "|" + propertyInfo.GetValue(this, null));

}

tw.Close();

}

public void SetProperty(PropertyInfo propertyInfo, object value)

{

switch(propertyInfo.PropertyType.Name)

{

case "Int32":

propertyInfo.SetValue(this, Convert.ToInt32(value), null);

break;

case "String":

propertyInfo.SetValue(this, value.ToString(), null);

break;

}

public int Age

{

get { return age; }

set { age = value; }

}

public string Name

{

get { return name; }

set { name = value; }

}

Okay, there's a lot of stuff, I know. But I will help you through the entire class. First of all, we have a couple of private fields, for holding information about our person. In the bottom of the class, we have the corresponding public properties which uses the private fields of course.

We also have a Load() method. It looks for the file settings.dat, and if it exists, it reads the entire file and places each line of it in an array of strings. Now, we run through each setting line, and splits it up into two parts: A property name and a value part. If both is present, we simply use the Type object to get the property with the property name, and then we set the value for it, using our own SetProperty method.

The SetProperty() method looks at the type of the property about to be changed, and then acts correspondingly. Right now, it only supports integers and strings, but as you probably realize, extending this support would be quite easy.

The Save() method gets an array of PropertyInfo instances, one for each of the defined properties on the Person class, and then uses a TextWriter to write each property, and its value, to the data file.

Now we just need some code to use this class. This small application will try to load the person from the settings file, and if it doesn't succeed, the user is prompted for the information:

class Program

{

static void Main(string[] args)

{

Person person = new Person();

person.Load();

if((person.Age > 0) && (person.Name != String.Empty))

{

Console.WriteLine("Hi " + person.Name + " - you are " + person.Age + " years old!");

}

else

{

Console.WriteLine("I don't seem to know much about you. Please enter the following information:");

Type type = typeof(Person);

PropertyInfo[] properties = type.GetProperties();

foreach(PropertyInfo propertyInfo in properties)

{

Console.WriteLine(propertyInfo.Name + ":");

person.SetProperty(propertyInfo, Console.ReadLine());

}

person.Save();

Console.WriteLine("Thank you! I have saved your information for next time.");

}

Console.ReadKey();

}

Everything here is pretty trivial, except for the part where we ask the user for information. Once again, we use Reflection, to get all the public properties of the Person class, and then ask for each of them.

As a reader exercise, I suggest that you extend the Person class to include more information. Simply add more properties to it, and you will see that this information gets saved and loaded too.