Interfaces

Syntax

interface IInterfaceName {\n  returnType MethodName(parameters);\n}

Examples

public interface IShape
{
    double Area();
    double Perimeter();
}

public class Rectangle : IShape
{
    public double Width, Height;

    public Rectangle(double width, double height)
    {
        Width = width;
        Height = height;
    }

    public double Area() => Width * Height;
    public double Perimeter() => 2 * (Width + Height);
}

IShape rect = new Rectangle(5, 3);
Console.WriteLine(rect.Area());  // 15

public interface ISwimmer { void Swim(); }
public interface IRunner { void Run(); }

public class Triathlete : ISwimmer, IRunner
{
    public void Swim() => Console.WriteLine("Swimming");
    public void Run() => Console.WriteLine("Running");
}

Triathlete athlete = new Triathlete();
athlete.Swim();
athlete.Run();

public interface IVehicle
{
    void Drive();

    void Honk() => Console.WriteLine("Beep beep!");  // has a body
}

public class Car : IVehicle
{
    public void Drive() => Console.WriteLine("Driving a car");
}

Car car = new Car();
car.Drive();
((IVehicle)car).Honk();  // uses the default implementation

public interface IPaymentMethod
{
    void Pay(double amount);
}

public class CreditCard : IPaymentMethod
{
    public void Pay(double amount) => Console.WriteLine($"Paid ${amount} with credit card");
}

public class PayPal : IPaymentMethod
{
    public void Pay(double amount) => Console.WriteLine($"Paid ${amount} with PayPal");
}

void ProcessPayment(IPaymentMethod method, double amount) => method.Pay(amount);

ProcessPayment(new CreditCard(), 50.0);
ProcessPayment(new PayPal(), 30.0);

Best practices

• Program to an interface rather than a concrete class (declare variables as IShape, not Rectangle) for more flexible, decoupled code
• Prefix interface names with I (IShape, IRepository) following standard C# naming conventions
• Keep interfaces small and focused (Interface Segregation Principle) - many specific interfaces beat one large, do-everything interface
• Use default interface methods sparingly, mainly for adding new functionality to an interface without breaking existing implementing classes

At a glance

Purpose

Applications on the .NET platform

File extension

.cs

Runs in

.NET runtime

Usually used with

.NET SDK and libraries

Specifications & further reading

• C# language reference

Related C# documentation