The abstract modifier is used to indicate that a class is incomplete and that it is intended to be used only as a base class. An abstract class differs from a non-abstract class in the following ways:

• An abstract class cannot be instantiated directly, and it is a compile-time error to use the new operator on an abstract class. While it is possible to have variables and values whose compile-time types are abstract, such variables and values will necessarily either be null or contain references to instances of non-abstract classes derived from the abstract types.
• An abstract class is permitted (but not required) to contain abstract members.
• An abstract class cannot be sealed.

When a non-abstract class is derived from an abstract class, the non-abstract class must include actual implementations of all inherited abstract members, thereby overriding those abstract members. [Example: In the example

abstract class A { public abstract void F(); } abstract class B: A { public void G() {} } class C: B { public override void F() { // actual implementation of F } }

the abstract class A introduces an abstract method F. Class B introduces an additional method G, but since it doesn't provide an implementation of F, B must also be declared abstract. Class C overrides F and provides an actual implementation. Since there are no abstract members in C, C is permitted (but not required) to be non-abstract. end example]