C# classes support inheritance, a popular object-oriented code reuse mechanism. When you write a class, you can optionally specify a base class. Your class will derive from this, meaning that everything in the base class will be present in your class, as well as any members you add.
Classes support only single inheritance. Interfaces offer a form of multiple inheritance. Value types do not support inheritance at all. One reason for this is that value types are not normally used by reference, which removes one of the main benefits of inheritance: runtime polymorphism. Inheritance is not necessarily incompatible with value-like behavior—some languages manage it—but it often has problems. For example, assigning a value of some derived type into a variable of its base type ends up losing all of the fields that the derived type added, a problem known as slicing. C# sidesteps this by restricting inheritance to reference types. When you assign a variable of some derived type into a variable of a base type, you’re copying a reference, not the object itself, so the object remains intact. Slicing is an issue only if the base class offers a method that clones the object, and doesn’t provide a way for derived classes to extend that (or it does, but some derived class fails to extend it).
Classes specify a base class using the syntax shown in Example 6-1—the base type appears after a colon
that follows the class name. This example assumes that a class called
SomeClass has ...