How to add an event to a class

Question

Say I have a class named Frog, it looks like:

public class Frog
{
     public int Location { get; set; }
     public int JumpCount { get; set; }


     public void OnJump()
     {
         JumpCount++;
     }

}

I need help with 2 things:

1. I want to create an event named Jump in the class definition.
2. I want to create an instance of the Frog class, and then create another method that will be called when the Frog jumps.

Answer 1

public event EventHandler Jump;
public void OnJump()
{
    EventHandler handler = Jump;
    if (null != handler) handler(this, EventArgs.Empty);
}

then

Frog frog = new Frog();
frog.Jump += new EventHandler(yourMethod);

private void yourMethod(object s, EventArgs e)
{
     Console.WriteLine("Frog has Jumped!");
}

Answer 2

@CQ: Why do you create a local copy pf Jump? Additionally, you can save the subsequent test by slightly changing the declaration of the event:

public event EventHandler Jump = delegate { };

public void OnJump()
{
    Jump(this, EventArgs.Empty);
}

Answer 3

Here is a sample of how to use a normal EventHandler, or a custom delegate. Note that ?. is used instead of . to insure that if the event is null, it will fail cleanly (return null)

public delegate void MyAwesomeEventHandler(int rawr);
public event MyAwesomeEventHandler AwesomeJump;

public event EventHandler Jump;

public void OnJump()
{
    AwesomeJump?.Invoke(42);
    Jump?.Invoke(this, EventArgs.Empty);
}

Note that the event itself is only null if there are no subscribers, and that once invoked, the event is thread safe. So you can also assign a default empty handler to insure the event is not null. Note that this is technically vulnerable to someone else wiping out all of the events (using GetInvocationList), so use with caution.

public event EventHandler Jump = delegate { };

public void OnJump()
{
    Jump(this, EventArgs.Empty);
}