The term abstraction is used to hide certain details and showing only essential information to the user. The MustInherit modifier indicates that the class or member has a missing or incomplete implementation.
MustInherit modifier can be used with classes, methods, properties, indexers, and events.MustInherit modifier is used in a class declaration, it is intended only to be a base class of other classes, and it cannot be instantiated on its own.MustOverride must be implemented by non-abstract classes that derive from the abstract class.MustOverride modifier in a method or property declaration to indicate that the method or property does not contain implementation.Let's have a look at the following simple example.
Public MustInherit Class Shape
Public MustOverride Function CalculateArea() As Double
End Class
We have declared the Shape class as an abstract. It contains a single method CalculateArea(), which is also abstract. So it means that we now need to inherit the Shape class and provide the implementation for the CalculateArea() method.
Public Class Circle
Inherits Shape
Public Property Radius As Double
Public Sub New(ByVal rad As Double)
Radius = rad
End Sub
Public Overrides Function CalculateArea() As Double
Return (3.14) * Math.Pow(Radius, 2)
End Function
End Class
Public Class Rectangle
Inherits Shape
Public Property Height As Double
Public Property Width As Double
Public Sub New(ByVal h As Double, ByVal w As Double)
Height = h
Width = w
End Sub
Public Overrides Function CalculateArea() As Double
Return Height * Width
End Function
End Class
As you can see, we have provided an implementation for the CalculateArea() abstract method in both child classes Circle and Rectangle with their implementation by calculating the area of circle and rectangle respectively.
Now we can create Circle and Rectangle objects and assign them to Shape instances, but we cannot create an object of the Shape class because it is an abstract class.
Dim circle As Shape = New Circle(2.5)
Dim rectangle As Shape = New Rectangle(4.75, 6.25)
Console.WriteLine("The area of the circle is " & circle.CalculateArea())
Console.WriteLine("The area of the rectangle is " & rectangle.CalculateArea())
You can see that both objects can call the CalculateArea(), but the right version of the CalculateArea() method is not being determined at compile time but determined at runtime.
Let's run the above code, and you will see the following output.
The area of the circle is 19.625
The area of the rectangle is 29.6875