In C#, an interface is a contract that defines a set of methods, properties, events, or indexers that a class or struct must implement. Interfaces are used to achieve abstraction and polymorphism in object-oriented programming. They allow different classes to implement the same set of methods, promoting a consistent API across different types.

Key Features of Interfaces

1. No Implementation: Interfaces cannot contain any implementation (i.e., no method bodies). They only define the signatures.
2. Multiple Inheritance: A class can implement multiple interfaces, allowing for a form of multiple inheritance.
3. Polymorphism: Interfaces allow for polymorphism, meaning that different classes can be treated as instances of the same interface type.

Defining an Interface

Here’s how you can define an interface in C#:

csharp
1public interface IAnimal
2{
3    void Speak(); // Method signature
4    string Name { get; set; } // Property signature
5}

Implementing an Interface

A class can implement an interface using the : syntax. Here’s an example:

csharp
1public class Dog : IAnimal
2{
3    public string Name { get; set; }
4
5    public Dog(string name)
6    {
7        Name = name;
8    }
9
10    public void Speak()
11    {
12        Console.WriteLine($"{Name} says Woof!");
13    }
14}
15
16public class Cat : IAnimal
17{
18    public string Name { get; set; }
19
20    public Cat(string name)
21    {
22        Name = name;
23    }
24
25    public void Speak()
26    {
27        Console.WriteLine($"{Name} says Meow!");
28    }
29}

Using the Interface

You can create instances of the classes that implement the interface and call their methods:

csharp
1class Program
2{
3    static void Main(string[] args)
4    {
5        IAnimal dog = new Dog("Buddy");
6        IAnimal cat = new Cat("Whiskers");
7
8        dog.Speak(); // Output: Buddy says Woof!
9        cat.Speak(); // Output: Whiskers says Meow!
10    }
11}

Interface with Multiple Implementations

You can implement the same interface in different classes:

csharp
1public class Bird : IAnimal
2{
3    public string Name { get; set; }
4
5    public Bird(string name)
6    {
7        Name = name;
8    }
9
10    public void Speak()
11    {
12        Console.WriteLine($"{Name} says Tweet!");
13    }
14}

Example with Multiple Interfaces

You can also implement multiple interfaces in a single class:

csharp
1public interface IMovable
2{
3    void Move();
4}
5
6public class Car : IAnimal, IMovable
7{
8    public string Name { get; set; }
9
10    public Car(string name)
11    {
12        Name = name;
13    }
14
15    public void Speak()
16    {
17        Console.WriteLine($"{Name} goes vroom!");
18    }
19
20    public void Move()
21    {
22        Console.WriteLine($"{Name} is moving!");
23    }
24}

Using the Class with Multiple Interfaces

csharp
1class Program
2{
3    static void Main(string[] args)
4    {
5        IAnimal myDog = new Dog("Buddy");
6        IMovable myCar = new Car("Tesla");
7
8        myDog.Speak(); // Output: Buddy says Woof!
9        myCar.Move();  // Output: Tesla is moving!
10    }
11}

Default Interface Methods (C# 8.0 and later)

Starting from C# 8.0, interfaces can contain default implementations for methods. This allows you to add new methods to an interface without breaking existing implementations.

csharp
1public interface IAnimal
2{
3    void Speak();
4
5    // Default implementation
6    void Eat()
7    {
8        Console.WriteLine("Eating...");
9    }
10}
11
12public class Dog : IAnimal
13{
14    public string Name { get; set; }
15
16    public Dog(string name)
17    {
18        Name = name;
19    }
20
21    public void Speak()
22    {
23        Console.WriteLine($"{Name} says Woof!");
24    }
25}

Using the Default Method

csharp
1class Program
2{
3    static void Main(string[] args)
4    {
5        IAnimal dog = new Dog("Buddy");
6        dog.Speak(); // Output: Buddy says Woof!
7        dog.Eat();   // Output: Eating...
8    }
9}

Summary

• Interfaces define a contract that classes must adhere to, promoting a consistent API.
• A class can implement multiple interfaces, allowing for flexible designs.
• Interfaces enable polymorphism, allowing you to treat different classes as instances of the same interface type.
• With C# 8.0 and later, interfaces can include default implementations, making it easier to evolve interfaces without breaking existing implementations.