When to Call Method after Property Change

Question

I have a User Control that contains a list of items and I raise an event when the currentIndex changes, also, when it changes, I must call two other methods two verify and change the appearance of the Control (change an Image and block/unblock some buttons).

What I want to know, mostly out of curiosity because it is already working, is when is it more appropriate to call these two methods?

Should I call them within the CurrentIndex property per se? Should I call them within the OnCurrentIndexChanged(...)? Should I handle the event within the class and do it there?

Answer 1

I'll assume you've implemented the standard event generating pattern and made OnCurrentIndexChanged protected virtual so that a derived class can override the method and alter the event generation and/or handling.

Unfortunately that requires reading tea leaves, why would anybody want to override the method? And more seriously, how could overriding the method break your control when they do? That's awfully hard to guess at for anybody that doesn't know the code well, not exactly easy for you either. The principle to apply here, used in the .NET framework code as well, is to do as little as possible. Just raise the event, nothing else. Which minimizes the odds of breakage when the derived class does something silly, but entirely common, like not calling base.OnCurrentIndexChanged.

The behavior of your controls is an implementation detail of your UserControl. So change their properties in your CurrentIndex property setter, then call OnCurrentIndexChanged(). Anybody that derives from your class can override that behavior, if necessary. And nothing goes wrong when they forget to call your OnCurrentIndexChanged() method. But do note that you need to make the control variables protected instead of private. So they can override the behavior, if they need to.

And don't hesitate to just not use a virtual method at all if this is too spooky for you. It's not common to have to accommodate hundreds of thousands of programmers with your controls :)