C# pattern to prevent an event handler hooked twice [duplicate]

Question

Duplicate of: How to ensure an event is only subscribed to once and Has an event handler already been added?

I have a singleton that provides some service and my classes hook into some events on it, sometimes a class is hooking twice to the event and then gets called twice. I'm looking for a classical way to prevent this from happening. somehow I need to check if I've already hooked to this event...

Answer 1

How about just removing the event first with -= , if it is not found an exception is not thrown

/// -= Removes the event if it has been already added, this prevents multiple firing of the event ((System.Windows.Forms.WebBrowser)sender).Document.Click -= new System.Windows.Forms.HtmlElementEventHandler(testii); ((System.Windows.Forms.WebBrowser)sender).Document.Click += new System.Windows.Forms.HtmlElementEventHandler(testii); 

Answer 2

Explicitly implement the event and check the invocation list. You'll also need to check for null:

using System.Linq; // Required for the .Contains call below:  ...  private EventHandler foo; public event EventHandler Foo {     add     {         if (foo == null || !foo.GetInvocationList().Contains(value))         {             foo += value;         }     }     remove     {         foo -= value;     } } 

Using the code above, if a caller subscribes to the event multiple times, it will simply be ignored.