Gränssnitt (introduktion) och implementering av flera gränssnitt

using System;

public interface IPlayable
{
  void Play();
}

public interface IHasDuration
{
  int DurationSeconds { get; }
}

public class Song : IPlayable, IHasDuration
{
  private readonly string _title;
  private readonly int _seconds;

  public Song(string title, int seconds)
  {
    _title = title;
    _seconds = seconds;
  }

  public int DurationSeconds { get { return _seconds; } }

  public void Play()
  {
    Console.WriteLine("Playing " + _title + " (" + _seconds + "s)");
  }
}

public class Program
{
  public static void Main(string[] args)
  {
    Song s = new Song("Hello", 5);
    s.Play();
    Console.WriteLine("Duration = " + s.DurationSeconds);
  }
}

using System;

public interface IPrintable
{
  void Print();
}

public class Report : IPrintable
{
  private readonly string _text;
  public Report(string text) { _text = text; }
  public void Print() { Console.WriteLine("Report: " + _text); }
}

public class Label : IPrintable
{
  private readonly string _text;
  public Label(string text) { _text = text; }
  public void Print() { Console.WriteLine("Label: " + _text); }
}

public class Program
{
  // Works with any IPrintable (polymorphism)
  static void PrintAll(IPrintable item)
  {
    item.Print();
  }

  public static void Main(string[] args)
  {
    PrintAll(new Report("Sales Q1"));
    PrintAll(new Label("Fragile"));
  }
}

using System;

public interface ILogger { void Write(string msg); }
public interface IFileWriter { void Write(string msg); }

public class DualWriter : ILogger, IFileWriter
{
  // Explicit implementations: only visible via the interface reference
  void ILogger.Write(string msg) { Console.WriteLine("[LOG] " + msg); }
  void IFileWriter.Write(string msg) { Console.WriteLine("[FILE] " + msg); }

  public void Demo()
  {
    Console.WriteLine("DualWriter ready");
  }
}

public class Program
{
  public static void Main(string[] args)
  {
    DualWriter dw = new DualWriter();
    dw.Demo();

    ILogger log = dw;
    IFileWriter file = dw;

    log.Write("started");
    file.Write("saved");
  }
}

using System;
using System.Collections.Generic;

public interface ITask
{
  void Run();
}

public class EmailTask : ITask
{
  public void Run() { Console.WriteLine("Send email"); }
}

public class BackupTask : ITask
{
  public void Run() { Console.WriteLine("Run backup"); }
}

public class Program
{
  public static void Main(string[] args)
  {
    List<ITask> tasks = new List<ITask>();
    tasks.Add(new EmailTask());
    tasks.Add(new BackupTask());

    foreach (ITask t in tasks)
    {
      t.Run(); // calls the right implementation
    }
  }
}