using System;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        A a = new A();

        // Noticed that Next() is called twice

        Console.WriteLine(a.Next(1));

        Console.WriteLine(a.Next(1));

        // Works

        var withCache = ((Func<int,int>)a.Next).AddCaching();

        withCache = new Func<int,int>(a.Next).AddCaching();

        withCache = ExtensionMethods.AddCaching<int,int>(a.Next);

        // Doesn't work :(

        // withCache = a.Next.AddCaching<int,int>();

        // Func<int,int> withCache = a.Next.AddCaching();

        // Notice that Next() is only called once

        Console.WriteLine(withCache(1));

        Console.WriteLine(withCache(1));

    }

}

​

public class A

{

    public int Next(int n)

    {

        Console.WriteLine("Called Next("+n+")");

        return n + 1;

    }

}

​

public static class ExtensionMethods

{

    public static Func<TKey,TVal> AddCaching<TKey,TVal>(this Func<TKey,TVal> fetcher)

    {

        var cache = new Dictionary<TKey, TVal>();

        return k =>

        {

            if (!cache.ContainsKey(k)) cache[k] = fetcher(k);

            return cache[k];

        };

    }

}