C# 0 The Complete Reference

Use Anonymous Methods and Lambda Expressions with Events

Anonymous methods and lambda expressions are especially useful when working with 

events because often the event handler is not called by any code other than the event 

handling mechanism. As a result, there is usually no reason to create a standalone method. 

Thus, the use of lambda expressions or anonymous methods can significantly streamline 

event handling code.

Since lambda expressions are now the preferred approach, we will start there. Here is an 

example that uses a lambda expression as an event handler:

// Use a lambda expression as an event handler.

using System;

// Declare a delegate type for an event.

delegate void MyEventHandler(int n);

// Declare a class that contains an event.

class MyEvent {

  public event MyEventHandler SomeEvent;

  // This is called to fire the event.

  public void OnSomeEvent(int n) {

    if(SomeEvent != null)

      SomeEvent(n);

  }

}

class LambdaEventDemo {

  static void Main() {

    MyEvent evt = new MyEvent();

    // Use a lambda expression as an event handler.

    evt.SomeEvent += (n) =>

      Console.WriteLine("Event received. Value is " + n);

    // Fire the event twice.

    evt.OnSomeEvent(1);

    evt.OnSomeEvent(2);

  }

}

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The output is shown here:

Event received. Value is 1

Event received. Value is 2

In the program, pay special attention to the way the lambda expression is used as an 

event handler, as shown here:

evt.SomeEvent += (n) =>

  Console.WriteLine("Event received. Value is " + n);

The syntax for using a lambda expression event handler is the same as that for using a 

lambda expression with any other type of delegate.

Although lambda expressions are now the preferred way to construct an anonymous 

function, you can still use an anonymous method as an event handler if you so choose. For 

example, here is the event handler from the previous example rewritten to use an 

anonymous method:

// Use an anonymous method as an event handler.

evt.SomeEvent += delegate(int n)  {

  Console.WriteLine("Event received. Value is" + n);

};

As you can see, the syntax for using an anonymous event handler is the same as that for any 

anonymous method.