Revision as of 19:14, 21 December 2007

Events in C#

An event is just an accessor (+=, -=) to a field holding a MulticastDelegate. The difference between a publicly accessible delegate field and an event are that you cannot overwrite the existing registered delegates, only append or remove. You also cannot reset the private event handler field to null from outside.

Declaration and what the Compiler does under the Surface

The typical declaration of an event in C# looks like this:

public event <type> <name>;

<type> is a derived type of MulticastDelegate which is a subtype of Delegate. Examples are Action<T>, MethodInvoker, EventHandler, ... etc. The compiler creates a private field with the same name and a pair of public accessors to add and remove delegates.

private <type> <name>;

public void add_<name>( <type> value){
   this.<name> = (<type>) Delegate.Combine(this.<name>, value);
}

public void remove_InvocationFailed(Action<Exception> value)
{
   this.<name> = (<type>) Delegate.Remove(this.<name>, value);
}

Quick Facts

Fireing an Event that has not been registered (==null) raises a NullReferenceException. (annoying)
Registering for the same event twice with the same delegate means that the delegate will be called twice.
A delegate that has been registered for the same event multiple times must also be unregistered multiple times.
Unregistering an event that has never been registered (using -= on a null event) has no effect.
There is no built-in mechanism for preventing event recursions. (see Preventing Recursive Events)

@@ Line 2: / Line 2: @@
 = Events in C# =
-An event is just an accessor (+=, -=) to a field holding a MulticastDelegate. The difference between a publicly accessible delegate field and an event are the following:
+An event is just an accessor (+=, -=) to a field holding a MulticastDelegate. The difference between a publicly accessible delegate field and an event are that you cannot overwrite the existing registered delegates, only append or remove. You also cannot reset the private event handler field to null from outside.
-* you cannot
-== Declaration ==
+== Declaration and what the Compiler does under the Surface ==
-  public event <delegate_type> <event_name>;
+The typical declaration of an event in C# looks like this:
-The compiler creates a private field with the same name.
+  public event <type> <name>;
-  private <delegate_type> <event_name>;
+<type> is a derived type of ''MulticastDelegate'' which is a subtype of ''Delegate''. Examples are ''Action<T>'', ''MethodInvoker'', ''EventHandler'', ... etc.
+The compiler creates a private field with the same name and a pair of public accessors to add and remove delegates.
+  private <type> <name>;
+ public void add_<name>( <type> value){
+    this.<name> = (<type>) Delegate.Combine(this.<name>, value);
+ }
+ public void remove_InvocationFailed(Action<Exception> value)
+ {
+    this.<name> = (<type>) Delegate.Remove(this.<name>, value);
+ }
 == Quick Facts ==