Question on .NET 1.1 C# Events Tutorial From MSDN

Question

I am looking at this event example http://msdn.microsoft.com/en-us/library/aa645739%28VS.71%29.aspx

This all makes sense to me except that the following line

public event ChangedEventHandler Changed;

What does this do?? Is this some sort of list of EVentCallbacks?? Why is new not used here??

EDIT: Why does this not need a NEW keyword??

public event ChangedEventHandler Changed;

Answer 1

This is your actual custom event to which you attach your event handlers.

Answer 2

What it's doing is associating an event named Changed with the ChangedEventHandler delegate.

Answer 3

It's declaring a field-like event, of type ChangedEventhandler, called Changed. Basically it's roughly equivalent to:

private ChangedEventHandler changedHandler;

public event ChangedEventHandler Changed
{
   add
   {
       lock(this)
       {
           changedHandler += value;
       }
   }
   remove
   {
       lock(this)
       {
           changedHandler -= value;
       }
   }
}

In other words, it creates an event which clients can subscribe to and unsubscribe from, and a variable to store those subscriptions. The event subscription/unsubscription code just combines/removes the given handler with the existing ones and stores the result in the field.

The result is that clients can subscribe to the event, e.g.

foo.Changed += ...;

and then when you raise the event, all the handlers are called.

See my article on events and delegates for more information.