C#. No13 - Interfaces

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An interface looks like a class, but has no implementation. The only thing it contains are definitions of events, indexers, methods and/or properties. The reason interfaces only provide definitions is because they are inherited by classes and structs, which must provide an implementation for each interface member defined.

So, what are interfaces good for if they don't implement functionality? They're great for putting together plug-n-play like architectures where components can be interchanged at will. Since all interchangeable components implement the same interface, they can be used without any extra programming. The interface forces each component to expose specific public members that will be used in a certain way.

Because interfaces must be implemented by derived classes and structs, they define a contract. For instance, if class foo implements the IDisposable interface, it is making a statement that it guarantees it has the Dispose() method, which is the only member of the IDisposable interface. Any code that wishes to use class foo may check to see if class foo implements IDisposable. When the answer is true, then the code knows that it can call foo.Dispose(). Listing 13-1 shows how to define an interface:

Defining an Interface: MyInterface.cs

interface IMyInterface
{
void MethodToImplement();
}

MyInterface.cs defines an interface named IMyInterface. A common naming convention is to prefix all interface names with a capital "I". This interface has a single method named MethodToImplement(). This could have been any type of method declaration with different parameters and return types. I just chose to declare this method with no parameters and a void return type to make the example easy. Notice that this method does not have an implementation (instructions between curly braces - {}), but instead ends with a semi-colon, ";". This is because the interface only specifies the signature of methods that an inheriting class or struct must implement. InterfaceImplementer.cs shows how this interface could be used.

Using an Interface: InterfaceImplementer.cs

class InterfaceImplementer : IMyInterface
{
static void Main()
{
InterfaceImplementer iImp = new InterfaceImplementer();
iImp.MethodToImplement();
}

public void MethodToImplement()
{
Console.WriteLine("MethodToImplement() called.");
}
}

The InterfaceImplementer class in InterfaceImplementer.cs implements the IMyInterface interface. Indicating that a class inherits an interface is the same as inheriting a class. In this case, the following syntax is used:

class InterfaceImplementer : IMyInterface

Now that this class inherits the IMyInterface interface, it must implement its members. It does this by implementing the MethodToImplement() method. Notice that this method implementation has the exact same signature, parameters and method name, as defined in the IMyInterface interface. Any difference will cause a compiler error. Interfaces may also inherit other interfaces. InterfaceInheritance.cs shows how inherited interfaces are implemented.

Interface Inheritance: InterfaceInheritance.cs

using System;

interface IParentInterface
{
void ParentInterfaceMethod();
}

interface IMyInterface : IParentInterface
{
void MethodToImplement();
}

class InterfaceImplementer : IMyInterface
{
static void Main()
{
InterfaceImplementer iImp = new InterfaceImplementer();
iImp.MethodToImplement();
iImp.ParentInterfaceMethod();
}

public void MethodToImplement()
{
Console.WriteLine("MethodToImplement() called.");
}

public void ParentInterfaceMethod()
{
Console.WriteLine("ParentInterfaceMethod() called.");
}
}

The code in InterfaceInheritance.cs contains two interfaces: IMyInterface and the interface it inherits, IParentInterface. When one interface inherits another, any implementing class or struct must implement every interface member in the entire inheritance chain. Since the InterfaceImplementer class in InterfaceInheritance.cs inherits from IMyInterface, it also inherits IParentInterface. Therefore, the InterfaceImplementer class must implement the MethodToImplement() method specified in the IMyInterface interface and the ParentInterfaceMethod() method specified in the IParentInterface interface.