using System;
using System.Drawing;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Reflection;

public class CachedFactory<K,T> : IDisposable where T : class
{
    private Dictionary<K,T> m_Objects;
    private readonly Func<K,T> m_Initializer;
    private readonly Func<T,T> m_Disposer;

    public CachedFactory(Func<K,T> initializer, Func<T,T> disposer)
    {
        if(disposer == null && typeof(IDisposable).IsAssignableFrom(typeof(T)))
        {
            disposer = obj =>
            {
                if(obj != null)
                {
                    ((IDisposable)obj).Dispose();
                }
                return null;
            };
        }

        m_Initializer = initializer;
        m_Disposer = disposer ?? (obj => null);
    }

    public T Create(K key)
    {
        T obj;
        if(m_Objects.TryGetValue(key, out obj))
            return obj;

        obj = m_Initializer(key);
        m_Objects[key] = obj;
        return obj;
    }

    public void Evict(K key)
    {
        m_Objects.Remove(key);
    }
}

public class BitmapCache : CachedFactory<string, Bitmap>
{
    public BitmapCache()
        : base(LoadBitmap, null) // IDisposable objects will be automatically disposed.
    {
    }

    private Bitmap LoadBitmap(string resourceName)
    {
        using(Stream stream = Assembly.GetExecutingAssembly().GetManifestResourceStream(resourceName))
        {
            return new Bitmap(stream);
        }

        return null;
    }
}


/*
 * BitmapCache bc = new BitmapCache();
 * ... = bc.Create("foo.bmp"); // loaded
 * ... = bc.Create("foo.bmp"); // cached
 * bc.Evict("foo.bmp");
 * ... = bc.Create("foo.bmp"); // loaded again
 */