Question

In following code, I want to extend the behaviour of a class by deriving/subclassing it, and make use of an event of the base class:

public class A {     public event EventHandler SomeEvent;      public void someMethod()     {         if(SomeEvent != null) SomeEvent(this, someArgs);     } }  public class B : A {     public void someOtherMethod()     {         if(SomeEvent != null) SomeEvent(this, someArgs); // << why is this not possible? //Error: The event 'SomeEvent' can only appear on the left hand side of += or -=  //(except when used from within the type 'A')     } } 

Why isn’t it possible?

And what is the common solution for this kind of situation?

Answer 1

The standard practice here is to have a protected virtual method OnSomeEvent on your base class, then call that method in derived classes. Also, for threading reasons you will want to keep a reference to the handler before checking null and calling it.

For an explanation of the why read Jon Skeet’s answer or the C# specification which describes how the compiler automatically creates a private field.

Here is one possible work around.

public class A {     public event EventHandler SomeEvent;      public void someMethod()     {         OnSomeEvent();     }      protected void OnSomeEvent()     {         EventHandler handler = SomeEvent;         if(handler != null)             handler(this, someArgs);     } }  public class B : A {     public void someOtherMethod()     {         OnSomeEvent();     } } 

Edit: Updated code based upon Framework Design Guidelines section 5.4 and reminders by others.