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Learning C# by Jesse Liberty

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Extending Interfaces

It is possible to extend an existing interface to add new methods or members, or to modify how existing members work. For example, you might extend ICompressible with a new interface, ILoggedCompressible, which extends the original interface with methods to keep track of the bytes saved. One such method might be called LogSavedBytes(). The following code creates a new interface named ILoggedCompressible that is identical to ICompressible except that it adds the method LogSavedBytes.

interface ILoggedCompressible : ICompressible
{
    void LogSavedBytes();
}

Classes are now free to implement either ICompressible or ILoggedCompressible, depending on whether they need the additional functionality. If a class does implement ILoggedCompressible, it must implement all the methods of both ILoggedCompressible and also ICompressible. Objects of that type can be cast either to ILoggedCompressible or to ICompressible.

Example 14-5 extends ICompressible to create ILoggedCompressible, and then casts the Document first to be of type IStorable, then to be of type ILoggedCompressible. Finally, the example casts the Document object to ICompressible. This last cast is safe because any object that implements ILoggedCompressible must also have implemented ICompressible (the former is a superset of the latter). This is the same logic that says you can cast any object of a derived type to an object of a base type (that is, if Student derives from Human, then all Students are Human, ...

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