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 writes Same! to the console.

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

After this code executes, is the EventHandler unregistered?

score 7 · Accepted Answer · answered Apr 03 '09 at 14:20

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;

score 0 · Answer 2 · answered Apr 03 '09 at 14:20

0

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.

answered Apr 03 '09 at 14:20

Lennaert

2,455
15
15

Actually it does. The compiler does it for you. – Samuel Apr 03 '09 at 14:21
I just tested it, and -- at least in my test -- after adding and removing the handler this way, it works. – Lennaert Apr 03 '09 at 14:40
This is simply shorthand, which the compiler still turns into a new EventHandler. – J. Steen Apr 03 '09 at 14:41
@Lennaert: Of course it works, I never said it didn't. I said your last sentence is wrong. – Samuel Apr 03 '09 at 14:43

Unregister events with new instance of the delegate

2 Answers2

Linked