Question

I have the following EventHandler:

private EventHandler<MyEventArgs> _myEventHandler;
public event EventHandler<MyEventArgs> MyEvent
{
  add { _myEventHandler += value; }
  remove { _myEventHandler -= value; }
}  

Could somebody explain the difference between the following snippets?
Snippet EventHandler (A):

//Snippet A:
if (_myEventHandler != null)
{
  _myEventHandler(new MyEventArgs());
}

Snippet BeginInvoke (B):

//Snippet B:
if (_myEventHandler != null)
{
  _myEventHandler.BeginInvoke(new MyEventArgs(), ar =>
  {
    var del = (EventHandler<MyEventArgs>)ar.AsyncState;
    del.EndInvoke(ar);
  }, _myEventHandler);
}

For clarification: What’s the difference between invoking an EventHandler “just as it is” and using BeginInvoke?

Answer 1

The BeginInvoke approach is async, meaning that it is raised on a different thread. This can be dangerous if people don’t expect it, and is pretty rare for events – but it can be useful.

Also, note that strictly speaking you should snapshot the event handler value – this is especially true if (via Begin*) you are dealing with threads.

var tmp = _myEventHandler;
if(tmp != null) {
    tmp(sender, args);
}

Also – note that your event subscription itself is not thread-safe; again, this only matters if you are dealing with multi-threading, but the inbuilt field-like event is thread-safe:

public event EventHandler<MyEventArgs> MyEvent; // <===== done; nothing more

The issues avoided here are:

• with the snapshot, we avoid the risk of the last subscriber unsubscribing between the null-check and the invoke (it does mean they might get an event they didn’t expect, but it means we don’t kill the raising thread)
• with the field-like event change we avoid the risk of losing subscriptions / unsubscriptions when two threads are doing this at the same time