How to invoke an event automatically when a function is called?

Question

I have this kind of code:

public class Foo { public SomeHandler OnBar; public virtual void Bar() { } }

Foo is a base class and therefor other classes might inherit from it.
I would like the OnBar event to always be fired when Bar() is called even if it's not called explicitly inside Bar.
How can it be done?

Quartermeister · Accepted Answer · 2010-07-11 15:06:20Z

A common pattern is to have a non-virtual method that will do what you want that calls a virtual method. Subclasses can override the inner method to change the functionality, but the public method can be non-virtual on always raise the event first.

public class Foo { public SomeHandler OnBar; public void Bar() { if (OnBar != null) { OnBar(this, EventArgs.Empty); } BarImpl(); } protected virtual void BarImpl() { } }

Basically an implementation of the template method pattern + 1
So this is the best way? In C++ I would use bind() to create a new function out of the event calling function and the original function.

Vilx- · Accepted Answer · 2010-07-11 15:05:32Z

Short answer: you can't. Not with what Microsoft gives you out of the box.

That said, take a look at "aspect oriented programming" in .NET. Google that, you might get something useful.

Added: The standard way would be to raise the event in the Bar() method and then require all derived classes to call the base implementation. But you can't enforce it.

So in which way would you "require" that the device class calls the base implementation? In the coding guidelines or the unit tests?
@carllacan - Woohoo! Congratulations, you found a 13 years old answer! :) Let's see... I guess that coding guidelines are the right place. Can't really enforce it in the unit tests either. That said, I like the approach in the accepted answer and would probably do that myself.

Itay Karo · Accepted Answer · 2010-07-11 15:07:18Z

1

first of all the code you provided will not compile.
virtual functions must have a body.

To ensure the event is fired you can do something like this.

public class Foo { public SomeHandler OnBar; public void Bar() { OnBar(); (check for nulls) ProtectedBar(); } protected virtual ProtectedBar() { } }

answered Jul 11, 2010 at 15:07

Itay Karo

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3 Comments

the_drow Over a year ago

You are correct. However that doesn't matter in this case. It was just to demonstrate that the function can be overriden. I don't know why you were downvoted. I upvoted to balance.

Dan Puzey Over a year ago

The code you've provided won't compile either - your virtual function has no return type and your comment isn't a comment.

Itay Karo Over a year ago

Dan Puzey: just followed the question style :P

mverardo · Accepted Answer · 2010-07-11 15:28:24Z

As Vilx- said, I guess the better way to do this kind of stuff is to use Aspect Oriented Programming.

This would help you with the "entangled code" (ie. calling an event in a method that shouldn't have this responsibility) and "scattered code" (ie. calling the same event in a lot of methods, duplicating your code) problems.

You should take a look at postsharp, it has a free community edition.

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