Question

How to define a must inherit class? in C#

Answer 1

You mark the class as abstract (this is the C# analogue to the VB.NET Must Inherit).

This will ensure it can’t be instantiated directly.

From the linked MSDN article:

The abstract modifier indicates that the thing being modified has a missing or incomplete implementation. The abstract modifier can be used with classes, methods, properties, indexers, and events. Use the abstract modifier in a class declaration to indicate that a class is intended only to be a base class of other classes. Members marked as abstract, or included in an abstract class, must be implemented by classes that derive from the abstract class.

(emphasis mine)