Chapter 4: Inheritance

Implementation versus Interface Inheritance

In object-oriented programming, there are two distinct types of inheritance: implementation inheritance and interface inheritance.

• Implementation inheritance means that a type derives from a base type, taking all the base type’s member fields and functions. With implementation inheritance, a derived type adopts the base type’s implementation of each function, unless it is indicated in the definition of the derived type that a function implementation is to be overridden. This type of inheritance is most useful when you need to add functionality to an existing type, or where a number of related types share a significant amount of common functionality. A good example of this comes in the Windows Forms classes, which are discussed in Chapter 28, “Windows Forms,” along with the base class System.Windows.Forms.Control, which provides a very sophisticated implementation of a generic Windows control, and numerous other classes such as System.Windows.Forms.TextBox and System.Windows.Forms.ListBox that are derived from Control and override functions or provide new functions to implement specific types of control.

• Interface inheritance means that a type inherits only the signatures of the functions but does not inherit any implementations. This type of inheritance is most useful when you want to specify that a type makes certain features available. For example, certain types can indicate that they provide a resource cleanup method called Dispose() by deriving from an interface, System.IDisposable (see Chapter 11, “Memory Management and Pointers”). Because the way that one type cleans up resources is likely to be very different from the way that another type cleans up resources, there is no point in defining any common implementation, so interface inheritance is appropriate here. Interface inheritance is often regarded as providing a contract: By deriving from an interface, a type is guaranteed to provide certain functionality to clients.

Traditionally, languages such as C++ have been very strong on implementation inheritance. Indeed, implementation inheritance has been at the core of the C++ programming model. On the other hand, Visual Basic 6 did not support any implementation inheritance of classes but did support interface inheritance thanks to its underlying COM foundations.

C# has both implementation and interface inheritance. There is arguably no preference, because both types of inheritance are fully built into the language from the ground up. This makes it easy for you to choose the best architecture for your solution.

Multiple Inheritance

Some languages such as C++ support what is known as multiple inheritance, in which a class derives from more than one other class. The benefits of using of multiple inheritance are debatable: On the one hand, there is no doubt that it is possible to use multiple inheritance to write extremely sophisticated, yet compact, code, as demonstrated by the C++ ATL library. On the other hand, code that uses multiple implementation inheritance is often difficult to understand and debug (a point that is equally well demonstrated by the C++ ATL library). As mentioned, making it easy to write robust code was one of the crucial design goals behind the development of C#. Accordingly, C# does not support multiple implementation inheritance. It does, however, allow types to be derived from multiple interfaces. This means that a C# class can be derived from one other class, and any number of interfaces. Indeed, we can be more precise: Thanks to the presence of System.Object as a common base type, every C# class (except for Object) has exactly one base class, and may additionally have any number of base interfaces.

Structs and Classes

Chapter 3 distinguishes between structs (value types) and classes (reference types). One restriction of using a struct is that structs do not support inheritance, beyond the fact that every struct is automatically derived from System.ValueType. In fact, we should be more careful. It’s true that it is not possible to code a type hierarchy of structs; however, it is possible for structs to implement interfaces. In other words, structs don’t really support implementation inheritance, but they do support interface inheritance. Indeed, we can summarize the situation for any types that you define as follows:

• Structs are always derived from System.ValueType. They can also be derived from any number of interfaces.

• Classes are always derived from one other class of your choosing. They can also be derived from any number of interfaces.