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// Ack! Illegal to allocate interface types.

static void Main(string[] args)

{

IPointy p = new IPointy();  // Compiler error!

}

Interfaces do not bring much to the table until they are implemented by a class or structure.

Here, IPointy is an interface that expresses the behavior of “having points.” The idea is simple:

some classes in the shapes hierarchy have points (such as the Hexagon), while others (such as the

Circle) do not.

Implementing an Interface

When a class (or structure) chooses to extend its functionality by supporting interface types, it does

so using a comma-delimited list in the type definition. Be aware that the direct base class must be

the first item listed after the colon operator. When your class type derives directly from System.

Object, you are free to simply list the interface(s) supported by the class, as the C# compiler will

extend your types from System.Object if you do not say otherwise. On a related note, given that

structures always derive from System.ValueType (see Chapter 4 for full details), simply list each

interface directly after the structure definition. Ponder the following examples: