C#でデザインパターン「Observerパターン」

using System;
using System.Collections.Generic;

// サブジェクトインターフェース
public interface IObservable<T>
{
    IDisposable Subscribe(IObserver<T> observer);
}

// オブザーバーインターフェース
public interface IObserver<T>
{
    void OnNext(T value);
    void OnError(Exception error);
    void OnCompleted();
}

// サブジェクトクラス
public class Subject<T> : IObservable<T>
{
    private List<IObserver<T>> observers = new List<IObserver<T>>();

    public IDisposable Subscribe(IObserver<T> observer)
    {
        observers.Add(observer);
        return new Unsubscriber<T>(observers, observer);
    }

    public void Notify(T value)
    {
        foreach (var observer in observers)
        {
            observer.OnNext(value);
        }
    }

    public void NotifyError(Exception error)
    {
        foreach (var observer in observers)
        {
            observer.OnError(error);
        }
    }

    public void NotifyCompleted()
    {
        foreach (var observer in observers)
        {
            observer.OnCompleted();
        }
    }
}

// 登録解除クラス
public class Unsubscriber<T> : IDisposable
{
    private List<IObserver<T>> observers;
    private IObserver<T> observer;

    public Unsubscriber(List<IObserver<T>> observers, IObserver<T> observer)
    {
        this.observers = observers;
        this.observer = observer;
    }

    public void Dispose()
    {
        if (observer != null && observers.Contains(observer))
        {
            observers.Remove(observer);
        }
    }
}

// オブザーバークラス
public class Observer<T> : IObserver<T>
{
    public void OnNext(T value)
    {
        Console.WriteLine($"Received value: {value}");
    }

    public void OnError(Exception error)
    {
        Console.WriteLine($"Received error: {error.Message}");
    }

    public void OnCompleted()
    {
        Console.WriteLine("Completed");
    }
}

// 使用例
public class Example
{
    public static void Main(string[] args)
    {
        var subject = new Subject<int>();
        var observer = new Observer<int>();

        subject.Subscribe(observer);

        subject.Notify(1);
        subject.Notify(2);
        subject.NotifyError(new Exception("An error occurred"));
        subject.NotifyCompleted();
    }
}

Received value: 1
Received value: 2
Received error: An error occurred
Completed