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

One reason that I could think of is that the actual creation of new instances would be handled by another public class in the same assembly. This would force that you create the instance through this other class - possibly some sort of a factory pattern implementation.

Answer 2

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.

Answer 3

It's public because it's used as a base class for the triggers that ship with WPF (Trigger, MultiTrigger, EventTrigger, DataTrigger etc). It it wasn't public then you wouldn't be able to flag these classes as public.

The constructors are internal because they don't intend for you to use it yourself. I'd guess you're suppose to derive from one of the classes mentioned above.