Abstract classes
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. In the example abstract class A abstract class B: A class C: B 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.
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