Question

Three related idioms: event, delegate, event-handler. I always get confused by who is “added” to who.

event += handler
event += delegate
handler += delegate

From what I know:

• delegate: a pointer to a function with a known signature.
• event-handler: a delegate which is registered to an event. Basically, is it the same as a delegate?
• event: a list of delegates\event-handlers which are executed when the event is invoked using event()

What confuses me more is this signature in MSDN:

public delegate void EventHandler(Object sender, EventArgs e)

Answer 1

Here is my summary (please correct me if I’m wrong):

• delegate is a pointer to a method (instance\static)

• eventHandler is a delegate with a specific signature (sender, eventArgs)

• event is an abstraction of accessing a delegate of any type, but it’s usually an eventHandler by convention

  //We declare delegates as a new type outside of any class scope (can be also inside?)
  
      public delegate retType TypeName (params...)
  
  //Here we assign
  
      public TypeName concreteDeleagteName = new TypeName (specificMethodName);
  
  //Declaring event
  //a. taken from http://stackoverflow.com/questions/2923952/explicit-event-add-remove-misunderstood
  
  private EventHandler _explicitEvent;
  public event EventHandler ExplicitEvent
  {
     add
     {
         if (_explicitEvent == null) timer.Start();
         _explicitEvent += value;
     }
     remove
     {
        _explicitEvent -= value;
        if (_explicitEvent == null) timer.Stop();
     }
  }
  
  //or: b.auto event - the compiler creates a "hidden" delegate which is bounded to this event
        public event TypeName eventName;
  

I want to recommend the great article Event Handling in .NET Using C#.

So we can only attach (eventName):

eventName += new TypeName (specificMethodName);

Which is equivalent to (_eventName is a delegate\eventHandler):

_eventName += new TypeName (specificMethodName);