Interfaces in C#

1. Interface = Pure Contract

public interface IDrivable
{
    void Start(); // No code, no access modifier
    void Stop();
}

public class SportsCar : IDrivable
{
    public void Start() { Console.WriteLine("VROOM!"); } // Must implement
    public void Stop() { Console.WriteLine("Screech!"); } // Must implement
}

2. Multiple Interfaces = Multiple Abilities

public interface IFlyable { void Fly(); }

public class FlyingCar : IDrivable, IFlyable // Multiple OK
{
    public void Start() { }
    public void Stop() { }
    public void Fly() { Console.WriteLine("Taking off"); }
}

1. Why Interfaces? Decouple Code

Write code that works with ANY object that has Start(). Don't care if it's Car, Boat, or Drone.

public void TestDrive(IDrivable vehicle)
{
    vehicle.Start(); // Works with SportsCar, Truck, Boat - anything IDrivable
    vehicle.Stop();
}

TestDrive(new SportsCar()); // OK
TestDrive(new Boat()); // OK if Boat : IDrivable

2. Interface vs Abstract Class

3. Explicit Implementation

Fix name clashes when 2 interfaces have same method.

public interface ICar { void Start(); }
public interface IComputer { void Start(); }

public class SmartCar : ICar, IComputer
{
    void ICar.Start() { Console.WriteLine("Engine start"); }
    void IComputer.Start() { Console.WriteLine("Boot OS"); }
}

SmartCar s = new SmartCar();
((ICar)s).Start(); // Engine start

4. Properties in Interface

public interface IDrivable
{
    int MaxSpeed { get; set; } // Must implement get and set
    void Start();
}