Unregister events with new instance of the delegate

Question

EventHandler a = new EventHandler(control_RegionChanged);
EventHandler b = new EventHandler(control_RegionChanged);

 if (a == b)
 {
     Console.WriteLine("Same!");
 }
 else
 {
     Console.WriteLine(a.GetHashCode() + " " + b.GetHashCode());
 }

This wirte "Same!" to the console.

control.RegionChanged += new EventHandler(control_RegionChanged);
control.RegionChanged -= new EventHandler(control_RegionChanged);

After this code the event is still unregistered? I think the answer is simply yes. But maybe this helps someone who has the same question.

Answer 1

Try using

control.RegionChanged += control_RegionChanged
control.RegionChanged -= control_RegionChanged

This should also work (from memory -- haven't really tested it). At least it doesn't create a new eventhandler-reference.

Answer 2

Yes; delegates are compared on the instance and MethodInfo; if those are the same, then it will work. The problem comes when trying to unsubscribe an anonymous method; in that case, you must keep a reference to the delegate in order to unsubscribe.

So:

This is fine:

control.SomeEvent += obj.SomeMethod;
//...
control.SomeEvent -= obj.SomeMethod;

But this is much riskier:

control.SomeEvent += delegate {Trace.WriteLine("Foo");};
//...
control.SomeEvent -= delegate {Trace.WriteLine("Foo");};

The correct process with anonymous methods is:

EventHandler handler = delegate {Trace.WriteLine("Foo");};
control.SomeEvent += handler;
//...
control.SomeEvent -= handler;