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

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

Create an Action instead of an event. Your class may look like:

public class MyClass
{
                // sender   arguments       <-----     Use this action instead of an event
     public Action<object, EventArgs> OnSomeEventOccured;

     public void SomeMethod()
     {
          if(OnSomeEventOccured!=null)
              OnSomeEventOccured(this, null);
     }

}

Answer 3

You really should handle this at the sink level and not the source level. That is, don't prescribe event handler logic at the event source - leave that to the handlers (the sinks) themselves.

As the developer of a service, who are you to say that sinks can only register once? What if they want to register twice for some reason? And if you are trying to correct bugs in the sinks by modifying the source, it's again a good reason for correcting these issues at the sink-level.

I'm sure you have your reasons; an event source for which duplicate sinks are illegal is not unfathomable. But perhaps you should consider an alternate architecture that leaves the semantics of an event intact.