internal constructor

Question

Yesterday I thought it would be nice to implement my own Trigger in a WPF app. I created a class MyTrigger which inherited TriggerBase. TriggerBase is a public abstract class. So inheritance isn't a problem. But the constructors inside this class are marked internal. The compiler throws an error because the is no valid constructor. Why does anyone create a public class but marks the constructors as internal?

Answer 1

If you want the class to be visible, but only allow it to be subclassed within your own assembly. The subclasses may have public constuctors themselves - or they may be accessed with a factory.

I can't comment on whether that's a good design decision for TriggerBase in WPF, but it's at least reasonable in some situations.