c# - Add delegate to event - thread safety

Question

It is possible to execute the following code from multiple threads simultaneously.

this._sequencer.Completed += OnActivityFinished;

Is it thread safe to add delegate to an event handler from multiple threads?

Is it thread safe to remove delegate to from event handler from multiple threads?

What is the simplest and maintainable way of making this thread safe?

Answer 1

If you don’t specify your own event add/remove handlers, the C# compiler generates this add handler (reconstructed by .NET Reflector):

public void add_MyEvent(EventHandler value)
{
    EventHandler handler2;
    EventHandler myEvent = this.MyEvent;
    do
    {
        handler2 = myEvent;
        EventHandler handler3 = (EventHandler) Delegate.Combine(handler2, value);
        myEvent = Interlocked.CompareExchange<EventHandler>(ref this.MyEvent, handler3, handler2);
    }
    while (myEvent != handler2);
}

and a remove handler that looks the same but with Delegate.Remove instead of Delegate.Combine.

Notice the use of Interlocked.CompareExchange? This prevents a race condition between updating the event’s backing field and reading from it. Thus, it is thread-safe.