Abstract Classes

Abstract classes and members are incomplete and must be implemented in a derived class.

• They cannot be directly instantiated.
• They cannot be static.
• They may contain constructors.

Abstract, Concrete, Virtual, and Static Members

Abstract classes may contain abstract, concrete, virtual, and static members:

• abstract members have no implementation.
  • They must be implemented in the derived class. Use the override keyword.
  • A field cannot be abstract.
  • They cannot be static.
• Concrete members have an implementation. They are inherited in the derived class like normal.
  • They may be re-implemented in the derived class with the new keyword.
  • They may be static.
• virtual members have an implementation. They are inherited in the derived class like normal.
  • They may be re-implemented in the derived class with the override keyword.
  • They cannot be static.
• static members have an implementation.
  • They may be re-implemented in the derived class with the new keyword.

Sealed

The sealed keyword prevents inheritance of a class, or of class members that were previously marked virtual. Because a sealed class cannot be the base class of a derived class, it also cannot be abstract.