What is the difference between a delegate type and eventhandler type?

Question

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

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


        

Answer 1

An "event" is really just shortcut for two methods that work with a delegate - the add and remove accessors. The compiler, by default, makes a delegate behind the event (if you don't write your own accessors).

When you call someEvent += aDelegate;, you're calling the event's add accessor. Normally, this is translated by the compiler into a delegate += call for a delegate with the same signature as the event - similar to how automatic properties automatically map to a backing field. This is why an event seems so similar to a delegate.

what confuses me more is this signature in MSDN: public delegate void EventHandler( Object sender, EventArgs e)

This signature is just a delegate signature. An event can, technically, use any delegate. However, by convention, it will always take two parameters - the first is the "sender" that raised the event, the second is a class that derives from EventArgs (like EventHandler and EventHandler<T>).

Answer 2

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);

Answer 3

The event has a delegate added to it which "points" to a handler.

So basically, when the event is raised, the collection of delegates it has, will be invoked, which as result will invoke handlers connected to those delegates.

//declare delegate
public delegate void EventHandler(  Object sender,  EventArgs e)

//create event based on delegate
public event EventHandler evHandler;

//function to attach to handler
public static void Handler(object sender, EventArgs e) {}

attach eventhandler function through delegate to event.
event += new EventHandler(Handler);