C# Interfaces

Overview

Interfaces define contracts that classes can implement. They specify what a class must do without specifying how it does it.

Basic Interface

public interface IDrawable
{
    void Draw();
    double GetArea();
}

public class Circle : IDrawable
{
    public double Radius { get; set; }
    
    public Circle(double radius) => Radius = radius;
    
    public void Draw() => Console.WriteLine($"Drawing circle with radius {Radius}");
    public double GetArea() => Math.PI * Radius * Radius;
}

Multiple Interfaces

public interface IMovable
{
    void Move(double x, double y);
}

public interface IResizable
{
    void Resize(double factor);
}

public class AdvancedShape : IDrawable, IMovable, IResizable
{
    public void Draw() => Console.WriteLine("Drawing shape");
    public double GetArea() => 100; // Implementation
    public void Move(double x, double y) => Console.WriteLine($"Moving to ({x}, {y})");
    public void Resize(double factor) => Console.WriteLine($"Resizing by {factor}");
}

Default Interface Methods (C# 8.0+)

public interface ILogger
{
    void Log(string message) => Console.WriteLine($"Log: {message}");
    void LogError(string error) => Console.WriteLine($"Error: {error}");
}

Practical Usage

public class DrawingManager
{
    public void DrawAll(List<IDrawable> shapes)
    {
        foreach (var shape in shapes)
        {
            shape.Draw();
        }
    }
}