Generic Types vs Abstract class/Interfaces

Question

Suppose we are creating a generic control in .NET. E.g. a tree. I don't understand why people use this generic type definition

Control<T>

when in Object Oriented Programming I can use an abstract class or an interface:

Control<IItem> or Control<BaseClass>

So the only thing to do is that, their types must derive from that base class or implement the interface. Does it mean, that generic types are more convenient, because you I don't have to implement or inherit anything?

Answer 1

Because in your case of a tree control, defining a generic Tree control would mean that Tree items can be of any type (you can also add certain constrains).

When instantiating a control, you would of course have to declare your item type (like in your second code examples with IItem and BaseClass).

If your Tree control wouldn't be a generic type, you would have to create several controls for each item type.

Why not just use interface/abstractBase type?
If you would just use an interface/abstract as your Item concrete class, you would be constrained by it's definition. You'd only see it's properties and methods. With generic Tree control and whatever item type, you're still able to access all item's properties and methods regardless of its interface implementation or parent class inheritance...