By default, all classes can serve as base classes in Visual Basic .NET. However, you can use two class-level modifiers, called inheritance modifiers, to modify that behavior:
NotInheritable— Prevents a class being used as a base class.
MustInherit— Indicates that the class is intended for use as a base class only.
Note that objects of MustInherit classes cannot be created directly; they can be created only as base class instances of a derived class.
We'll see both of these modifiers in this chapter.