Re: Consuming your own events.

> the guideline clearly state that non-sealed classes that expose events
> should implement a:
> protected virtual void OnTheEvent( EventArgs e ) for every public
> event.

Guidelines are just that, guidelines. They're not always right, and over
time the get more wrong.

The problem with the OnXxx notation is that it's overloaded with meaning.
Some class libraries put the event raising on it, others use it for
handlers. The intent of the overriding mechanism can only be described in
the documentation, and calling base.OnXxx is up to the developer. It's in my
opinion quite a poor design.

If you want to cancel an event, have a CancelableEventArgs with a bool
Cancel property, raise it through the RaiseXxx method. Have all the handlers
in the form HandleXxx. Adopt the Xxxing / Xxxed notation to provide for
ordering. Provide an optional OnXxx that is called before the event and can
cancel it, having essentially the semantics of an event handler attached to
the instance of a type, something that is quite common in WPF where attached
events are docked to a type through the OnXxx method.

I find the code semantics of "ea.Cancel = true" much better than the
semantics of not calling the base class as a way to cancel an event.

If your object is in an undeterministic state if throwing when the event is
raised, so will it with overridden OnXxx that forgets to call the base class
implementation.

I've seen way too many issues all around the semantics of calling
base.OnXxx, where it is sometimes needed, sometimes not, and never visible
in the code, to find the common meme on event raising / handling to be
either sufficient or in any way accurate.

It's a bit like the Begin / End async handling where people were quite wrong
for years on the necessity to call End because the semantics were left to
documentation.


--
SerialSeb

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