ความแปรผัน (in/out) กับอินเทอร์เฟซและผู้รับมอบหมาย

ภาพรวมความแปรผันของชนิดข้อมูล

ตัวอย่าง Covariance

using System;
using System.Collections.Generic;

public class Animal { public virtual string Name() { return "animal"; } }
public class Cat : Animal { public override string Name() { return "cat"; } }

public class Program
{
  static void PrintAll(IEnumerable<Animal> animals)
  {
    foreach (Animal a in animals)
    {
      Console.WriteLine(a.Name());
    }
  }

  public static void Main(string[] args)
  {
    List<Cat> cats = new List<Cat>();
    cats.Add(new Cat());
    // IEnumerable<Cat> -> IEnumerable<Animal> is allowed (out T)
    PrintAll(cats);
  }
}

ตัวอย่าง Contravariance

using System;
using System.Collections.Generic;

public class Animal { public string N; public Animal(string n){ N=n; } }
public class Cat : Animal { public Cat(string n) : base(n) {} }

// Comparer can compare any Animals
public class AnimalNameComparer : IComparer<Animal>
{
  public int Compare(Animal x, Animal y)
  {
    return string.Compare(x == null ? "" : x.N, y == null ? "" : y.N, StringComparison.Ordinal);
  }
}

public class Program
{
  public static void Main(string[] args)
  {
    List<Cat> cats = new List<Cat>();
    cats.Add(new Cat("Miso"));
    cats.Add(new Cat("Ada"));
    // IComparer<Animal> can be used as IComparer<Cat> (in T)
    cats.Sort(new AnimalNameComparer());
    foreach (Cat c in cats) Console.WriteLine(c.N);
  }
}

ความแปรผันของ Delegate

using System;

public class Animal { public virtual string Speak(){ return "???"; } }
public class Cat : Animal { public override string Speak(){ return "meow"; } }

public class Program
{
  static Animal MakeAnimal() { return new Animal(); }
  static void HandleAnimal(Animal a) { Console.WriteLine(a.Speak()); }

  public static void Main(string[] args)
  {
    // Func<Cat> can be used where Func<Animal> is expected? Yes, because TResult is out.
    Func<Animal> maker = new Func<Cat>(delegate { return new Cat(); });
    Console.WriteLine(maker().Speak());

    // Action<Animal> can be used where Action<Cat> is expected? Yes, because T is in.
    Action<Cat> catSink = new Action<Animal>(HandleAnimal);
    catSink(new Cat());
  }
}

อินเทอร์เฟซแบบแปรผันที่กำหนดเอง

using System;

// Producer-only interface: out T
public interface ISource<out T>
{
  T Get();
}

// Consumer-only interface: in T
public interface ISink<in T>
{
  void Put(T item);
}

public class Animal { public string N; }
public class Cat : Animal { }

public class CatSource : ISource<Cat>
{
  public Cat Get() { return new Cat(); }
}

public class AnimalSink : ISink<Animal>
{
  public void Put(Animal a) { Console.WriteLine("Got animal"); }
}

public class Program
{
  public static void Main(string[] args)
  {
    ISource<Animal> animals = new CatSource(); // out T allows upcast
    ISink<Cat> cats = new AnimalSink();        // in T allows down-assign
    animals.Get();
    cats.Put(new Cat());
  }
}

เคล็ดลับเรื่องความแปรผัน

คีย์เวิร์ด Covariance

ทบทวน