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.

Abstract classes have the following features:

  • An abstract class cannot be instantiated.

  • An abstract class may contain abstract methods and accessors.

  • It is not possible to modify an abstract class with the sealed modifier because the two modifers have opposite meanings. The sealed modifier prevents a class from being inherited and the abstract modifier requires a class to be inherited.

  • A non-abstract class derived from an abstract class must include actual implementations of all inherited abstract methods and accessors.

Use the abstract modifier in a method or property declaration to indicate that the method or property does not contain implementation.

Abstract methods have the following features:

  • An abstract method is implicitly a virtual method.

  • Abstract method declarations are only permitted in abstract classes.

  • Because an abstract method declaration provides no actual implementation, there is no method body; the method declaration simply ends with a semicolon and there are no curly braces ({ }) following the signature. For example:

  • public abstract void MyMethod();
  • The implementation is provided by an overriding method, which is a member of a non-abstract class.

  • It is an error to use the static or virtual modifiers in an abstract method declaration.

Abstract properties behave like abstract methods, except for the differences in declaration and invocation syntax.

  • It is an error to use the abstract modifier on a static property.

  • An abstract inherited property can be overridden in a derived class by including a property declaration that uses the override modifier.

Source: MSDN