MustInherit
Specifies that a class can be used only as a base class and that you cannot create an object directly from it.
Remarks
The purpose of a base class (also known as an abstract class) is to define functionality that is common to all the classes derived from it. This saves the derived classes from having to redefine the common elements. In some cases, this common functionality is not complete enough to make a usable object, and each derived class defines the missing functionality. In such a case, you want the consuming code to create objects only from the derived classes. You use MustInherit on the base class to enforce this.
Another use of a MustInherit class is to restrict a variable to a set of related classes. You can define a base class and derive all these related classes from it. The base class does not need to provide any functionality common to all the derived classes, but it can serve as a filter for assigning values to variables. If your consuming code declares a variable as the base class, Visual Basic allows you to assign only an object from one of the derived classes to that variable.
The .NET Framework defines several MustInherit classes, among them Array, Enum, and ValueType. ValueType is an example of a base class that restricts a variable. All value types derive from ValueType. If you declare a variable as ValueType, you can assign only value types to that variable.
Rules
Declaration Context. You can use MustInherit only in a Class statement.
Combined Modifiers. You cannot specify MustInherit together with NotInheritable in the same declaration.
Example
The following example illustrates both forced inheritance and forced overriding. The base class shape defines a variable acrossLine. The classes circle and square derive from shape. They inherit the definition of acrossLine, but they must define the function area because that calculation is different for each kind of shape.
Public MustInherit Class shape
Public acrossLine As Double
Public MustOverride Function area() As Double
End Class
Public Class circle : Inherits shape
Public Overrides Function area() As Double
Return Math.PI * acrossLine
End Function
End Class
Public Class square : Inherits shape
Public Overrides Function area() As Double
Return acrossLine * acrossLine
End Function
End Class
Public Class consumeShapes
Public Sub makeShapes()
Dim shape1, shape2 As shape
shape1 = New circle
shape2 = New square
End Sub
End Class
You can declare shape1 and shape2 to be of type shape. However, you cannot create an object from shape because it lacks the functionality of the function area and is marked MustInherit.
Because they are declared as shape, the variables shape1 and shape2 are restricted to objects from the derived classes circle and square. Visual Basic does not allow you to assign any other object to these variables, which gives you a high level of type safety.
Usage
The MustInherit modifier can be used in this context:
See Also
Reference
Visual Basic Language Keywords