Lesson 21: Anonymous Methods

In Lesson 14: Introduction to Delegates, you learned about delegates and how they enable you to connect handlers to events. For C# v2.0, there is a new language feature, called anonymous methods, that are similar to delegates, but require less code. While you learn about anonymous methods, we'll cover the following objectives:

• Understand the benefits of anonymous methods
• Learn how to implement an anonymous method
• Implement anonymous methods that use delegate parameters

How Do Anonymous Methods Benefit Me?

An anonymous method is a method without a name - which is why it is called anonymous. You don't declare anonymous methods like regular methods. Instead they get hooked up directly to events. You'll see a code example shortly.
To see the benefit of anonymous methods, you need to look at how they improve your development experience over using delegates. Think about all of the moving pieces there are with using delegates: you declare the delegate, write a method with a signature defined by the delegate interface, declare the event based on that delegate, and then write code to hook the handler method up to the delegate. With all this work to do, no wonder programmers, who are new to C# delegates, have to do a double-take to understand how they work.
Because you can hook an anonymous method up to an event directly, a couple of the steps of working with delegates can be removed. The next section shows you how this works.

Implementing an Anonymous Method

An anonymous method uses the keyword, delegate, instead of a method name. This is followed by the body of the method. Typical usage of an anonymous method is to assign it to an event. Listing 21-1 shows how this works.

Listing 21-1. Implementing an Anonymous Method

using System.Windows.Forms;

public partial class Form1 : Form
{
    public Form1()
    {
        Button btnHello = new Button();
        btnHello.Text = "Hello";

        btnHello.Click +=
            delegate
            {
                MessageBox.Show("Hello");
            };

        Controls.Add(btnHello);
    }
}

The code in Listing 21-1 is a Windows Forms application. It instantiates a Button control and sets its Text to "Hello". Notice the combine, +=, syntax being used to hook up the anonymous method. You can tell that it is an anonymous method because it uses the delegate keyword, followed by the method body in curly braces.
Essentially, you have defined a method inside of a method, but the body of the anonymous method doesn't execute with the rest of the code. Because you hook it up to the event, the anonymous method doesn't execute until the Click event is raised. When you run the program and click the Hello button, you'll see a message box that say's "Hello" - courtesy of the anonymous method.
Using Controls.Add, adds the new button control to the window. Otherwise the window wouldn't know anything about the Button and you wouldn't see the button when the program runs.

Using Delegate Parameters with Anonymous Methods

Many event handlers need to use the parameters of the delegate they are based on. The previous example didn't use those parameters, so it was more convenient to not declare them, which C# allows. Listing 21-2 shows you how to use parameters if you need to.

Listing 21-2. Using Parameters with Anonymous Methods

using System;
using System.Windows.Forms;

public partial class Form1 : Form
{
    public Form1()
    {
        Button btnHello = new Button();
        btnHello.Text = "Hello";

        btnHello.Click +=
            delegate
            {
                MessageBox.Show("Hello");
            };

        Button btnGoodBye = new Button(); 
        btnGoodBye.Text = "Goodbye";
        btnGoodBye.Left = btnHello.Width + 5; 
        btnGoodBye.Click += 
            delegate(object sender, EventArgs e) 
            { 
                string message = (sender as Button).Text;
                MessageBox.Show(message); 
            };

        Controls.Add(btnHello);
        Controls.Add(btnGoodBye);
    }
}

The bold parts of Listing 21-2 show another Button control added to the code from Listing 21-1. Besides changing the text, btnGoodBye is moved to the right of btnHello by setting it's Left property to 5 pixels beyond the right edge of btnHello. If we didn't do this, btnGoodBye would cover btnHello because both of their Top and Left properties would default to 0.
Beyond implementation details, the real code for you to pay attention to is the implementation of the anonymous method. Notice that the delegate keyword now has a parameter list. this parameter list must match the delegate type of the event that the anonymous method is being hooked up to. The delegate type of the Click event is EventHandler, which has the following signature:

public delegate void EventHandler(object sender, EventArgs e);

Notice the EventHandler parameters. Now, here's how the Button control's Click event is defined:

public event EventHandler Click;

Notice that the delegate type of the Click event is EventHandler. This is why the anonymous method, assigned to btnGoodBye.Click in Listing 21-2, must have the same parameters as the EventHandler delegate.

Summary

Anonymous methods are a simplified way for you to assign handlers to events. They take less effort than delegates and are closer to the event they are associated with. You have the choice of either declaring the anonymous method with no parameters or you can declare the parameters if you need them.