Bring up a project, and go to Solution Explorer; right-click, select Add, and click on Class. Name the class GenericsClass; a simple generics class. Then, click on OK. When the Visual Studio message comes up, click on Yes.

For our purposes, you don't need any of the using System lines at the top, nor any of the comments underneath, so delete them. Your initial screen should look like Figure 1.1.1:

Now, let's put a <T> symbol after where it says public class GenericsClass, as follows:

public class GenericsClass<T>

This means that this single class can work equally well with several different data types. Next, enter the following beneath the open curly brace under the preceding line:

private T[] vals;

Enter the following comment directly above this line:

//generic array instance variable

In other words, this will operate equally well on doubles, decimals, integers, and so on.

Now, in the following line, enter the following:

public GenericsClass(T[] input)

As you can see, you can also make parameters that are generic like this one. This is a parameter, input is the name of it, and the type is T. So, it's a generic array.

Next, enter the following between a set of curly braces beneath the preceding line:

vals = input;

Of course, you should be able to display these values. so, enter the following line beneath the closed curly brace under the vals = input; line:

public string DisplayValues()

To display these values, you'll enter the following between a set of curly braces beneath the preceding line.

First, put in a string, as follows:

string str = null;

Next, declare the string and initialize the value to null.

Then, enter the following directly below this line:

foreach ( T t in vals)

As you can see, the foreach loop here is going to operate. The T object will be a different data type, depending on how we choose to make the object. The t variable, of course, is each specific value inside the vals array.

Next, you will enter the following between a set of curly braces beneath the preceding line:

str += $"<br>Value={t}";

Remember, we use the += operator to accumulate and <br> to push down to the next line. To get the value, of course, we will put in the t variable.

At the end, you want to return this, so you will type the following beneath the closed curly brace under the preceding line:

return str;

That's it. The final version of the GenericsClass.cs file for this chapter, including comments, is shown in the following code block:

 //<T> means this class can operate on many different data types
public class GenericsClass<T>
{
    //generic array instance variable
    private T[] vals;//array of T inputs
    public GenericsClass(T[] input)
    {
        //set value of instance variable
        vals = input;
    }
    public string DisplayValues()
    {
        string str = null;//create string to build up display
        foreach(T t in vals)
        {
            //actually accumulate stuff to be displayed
            str += $"<br>Value={t}";
        }
    //return string of outputs to calling code
    return str;
    }
}  

Notice that we have a single block of code; this will now operate on integers, doubles, and so on.

Now, let's take a look at Default.aspx. The only thing we really need to do at this time is to add a Button control. For this, go to Toolbox and grab a Button control from there. Drag and drop it below the line beginning with <form id=... (you can delete the <div> lines, as we won't be needing them). Change the text on the Button control to, for example, Display Values. Your complete Default.aspx file should look like the one shown in Figure 1.1.2:

Now, go to the Design view. Our very simple interface is shown in Figure 1.1.3:

Now, double-click on the Display Values button and go into Default.aspx.cs. Delete the Page_Load block. Next, between the set of curly braces beneath the line beginning with protected void Button1_Click..., enter the following:

GenericsClass<int> ints = new GenericsClass<int>(new int[] { 1, 2, 3, 4, 5 });

You can see in this line that we are basically initializing a collection of integers to their array.

Now, you can display this. So, for example, you can enter the following below this line:

sampLabel.Text += ints.DisplayValues();

Notice that the GenericsClass which we have constructed is operating in integers, but it can operate equally well on any other data type.

Now, to make the code efficiency more obvious, take both of the preceding lines, copy them (Ctrl + C) and paste them (Ctrl + V) beneath these and just change it to double, as follows:

GenericsClass<double> dubs = new GenericsClass<double>(new double[] {1, 2, 3, 4, 5});
sampLabel.Text = ints.DisplayValues();

We'll call this one dubs and change the name here to double: it's the same code, the same class, and the same generic class that you can operate on the doubles. Again, to emphasize this one more time, and to see that flexibility and code reuse is really the purpose here; that is, the ability to reuse code, we'll now take both of these new lines, copy and paste them below once more, and just change double to decimal, as follows:

GenericsClass<decimal> decs = new GenericsClass<decimal>(new decimal[] { 1, 2, 3, 4, 5 });
sampLabel.Text = ints.DisplayValues();

Let's call this one decs. Now, of course, if you want to make things a little more interesting, you can throw in some decimals:

GenericsClass<double> dubs = new GenericsClass<double>(new double[] { 1.0, -2.3, 3, 4, 5 });
sampLabel.Text = ints.DisplayValues();
GenericsClass<decimal> decs = new GenericsClass<decimal>(new decimal[] { 1, 2.0M, 3, 4, 5.79M });
sampLabel.Text = ints.DisplayValues();

Now, let's take a look. When you run this code and click on the Display Values button, your screen will look like the one shown in Figure 1.1.4:

Now, we will accumulate the input. So, in the following sampLabel.Text lines, we change the = sign to +=, as shown here:

GenericsClass<double> dubs = new GenericsClass<double>(new double[] { 1.0, -2.3, 3, 4, 5 });
sampLabel.Text += ints.DisplayValues();
GenericsClass<decimal> decs = new GenericsClass<decimal>(new decimal[] { 1, 2.0M, 3, 4, 5.79M });
sampLabel.Text += ints.DisplayValues();

Let's run it one more time. Click on the Display Values button and your screen will now look like the one shown in Figure 1.1.5:

The program is now working as expected.

So, the big idea of generics at this point is that you can define a generic class. This class can operate equally well on many different data types. For example, you can make a generic class that operates on integers as well as on doubles and decimals.

Now, we will go through it. So, to first step into it, hover your mouse over the T object in the following line in Generics Class.cs:

public GenericsClass(T[] input)

Here, T is essentially like a parameter, so it does have a certain value, which is expressed in the vals = input; line. The first time, T is used for integers. This is how you can step through this code. At the bottom of the screen, the values inside the array are displayed, as shown in Figure 1.1.6:

The t variable, as you can see in Figure 1.1.7, is an integer, and this is how it operates:

Notice also in the screenshot that it's a generics class with an <int> datatype.

The T object in the foreach(T t in vals) line right now represents an integer, and so on for the other data types. So, flexibility of code and reuse of code means that you will write less code. If not for generics, you would have to create individual classes to handle each different data type.

For review, the complete version of the Default.aspx.cs file for this chapter, including comments, is shown in the following code block:

//using is a directive
//System is a name space
//name space is a collection of features that our needs to run
using System;
//public means accessible anywhere
//partial means this class is split over multiple files
//class is a keyword and think of it as the outermost level of grouping
//:System.Web.UI.Page means our page inherits the features of a Page
public partial class _Default : System.Web.UI.Page
{
    protected void Button1_Click(object sender, EventArgs e)
    {
        //in each case below, GenericsClass<...> works equally well with
        //integers, doubles and decimals, among others
        GenericsClass<int> ints = new GenericsClass<int>(new int[] { 1, 2, 3, 4, 5 });
        sampLabel.Text = ints.DisplayValues();
        GenericsClass<double> dubs = new GenericsClass&lt;double>(new double[] { 1.0, -2.3, 3, 4, 5 });
        sampLabel.Text += ints.DisplayValues();
        GenericsClass<decimal> decs = new GenericsClass<decimal>(new decimal[] { 1, 2.0M, 3, 4, 5.79M });
        sampLabel.Text += decs.DisplayValues();
    }
} 

In this chapter, you learned about the basics of making a simple generic class, so that one class can operate on many different data types. A great benefit of generics is flexibility. You created a simple generics class that worked with different data types, made generic parameters, initialized a collection of integers to their array and displayed the results, and then changed the data types in the generics class to doubles and decimals.

In the next chapter, you will learn about generic methods, or methods that can operate on different data types. You will also learn about constraining a method in terms of the data types that it can operate on, so we'll add a concept called constraints.