Click here to download the sample source code for this Lists lab exercise 

Now that we have seen how Arrays work, let’s take a look at a more versatile C# object: the class.  This class is found in the System.Collections.Generic namespace.  I like to think of the List class as a fancy Array; it is very similar to an Array, with more functionality and it is more dynamic.  With a List object, you don’t have to define the size of the List ahead of time.  It can grow dynamically based on how many items you put into it.  Just that characteristic alone, makes it easier to use for a lot of C# programmers.  If you know ahead of time, how many items you need to store in a list, you may be able to use the Array, but if you are unsure, the List object may suit you better.  Let’s see an example.

Create a new web form called Default2.aspx in your Lab3 project.  Click here to watch an example video in case you forgot how to add a new web form.  Drag a Button onto the form and change the Text to “List of integers”.  Drag a TextBox onto the form and make it MultiLine.  Double click the Button to create the event handler and put the following:

 

List<int> mylist = new List<int>();

mylist.Add(33);
mylist.Add(55);
mylist.Add(10);
mylist.Add(509);

foreach (int myvar in mylist)
{
    TextBox1.Text += myvar.ToString() + "\r\n";
}

In the using statements at the top of the source code file, make sure that you have “using System.Collections.Generic” listed somewhere, otherwise you will get an error when you try to run the project and the compiler may complain it cannot find the List class.

To declare a List, I used the syntax List<int> mylist = new List<int>();  The data type is List and following the data type is an expression that tells C# what kind of type you plan to hold in the List.  In my case, my List hold int numbers so I placed that inside the < >.  Notice on the right side of the assignment operator = that I had to put parentheses after List<int>.  In the later lessons dedicated to coding and calling C# classes I talk more in detail about why this is necessary.

Putting items into the List is easy, you use the Add method mylist.Add(10);  Notice that I didn’t have to declare how big my List is, I can call the Add method as many times as I like.

To access the items in the List you can use a similar syntax to that of Arrays mylist[0] using the index number.  In the example above I used a slightly different approach which is the C# loop.  The foreach loop is a special kind of loop in C# that can easily loop through Collections.  Collections are special types that facilitate this kind of iteration.  The List class is a type of Collection.

With the syntax foreach (int myvar in mylist) we can talk about each part of the foreach syntax separately.  Inside the parentheses there are 4 parts to the foreach expression.  The first part specifies the data type “int” that we are planning to read from the Collection.  The second part “myvar” is simply a temporary variable that will hold the contents of the List each time the loop iterates.  The third part is the keyword “in”.  Finally, the fourth part is the name of the Collection that you want to loop through which in this case is “mylist”, the List class.  This loop will start at the beginning of the List starting with the first item that you added into the List and end with the last item which was added to the List.  I could have also used a for loop instead of the foreach loop:

 

for (int i = 0; i < mylist.Count; i++)
{
    TextBox1.Text += mylist[i].ToString() + "\r\n";
}