C# Interface

What Is an Interface?

An interface is like a completely abstract class. It contains only the signatures of methods and properties, never their bodies (in the basic case). A class that uses an interface promises to provide the actual code for every member listed. You can think of an interface as a to-do list that a class agrees to complete.

By convention, interface names in C# start with a capital letter I, for example IAnimal or IShape. This makes them easy to recognize in your code.

interface IAnimal
{
    void MakeSound();  // no body, and no access modifier
}

class Cat : IAnimal
{
    public void MakeSound()
    {
        Console.WriteLine("The cat says: meow");
    }
}

Rules for Interfaces

• You cannot create an object from an interface directly.
• A class that implements an interface must provide a body for every member.
• Interface members do not have an access modifier; they are public automatically.
• A class can implement more than one interface at the same time.

Implementing Multiple Interfaces

C# does not allow a class to inherit from more than one base class. However, a class can implement several interfaces at once by separating them with commas. This is how C# lets a class combine many sets of behaviour.

interface IFirst
{
    void MethodOne();
}

interface ISecond
{
    void MethodTwo();
}

class DemoClass : IFirst, ISecond
{
    public void MethodOne()
    {
        Console.WriteLine("Method one running");
    }
    public void MethodTwo()
    {
        Console.WriteLine("Method two running");
    }
}

Interfaces are ideal when unrelated classes need to share the same set of abilities. For example, a Bird and an Airplane are very different, but both could implement an IFlyable interface with a Fly method.