Memoize in C# inspired by http://inlehmansterms.net/2015/03/01/javascript-memoization/

Memoize.cs

using System;
using System.Collections.Generic;
using System.Linq;
using Newtonsoft.Json;

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            Func<int, int> factorial = null;
            factorial = Memoize<int, int>(n =>
            {
                Console.WriteLine($"Working for factorial {n}");
                if (n == 1)
                {
                    return 1;
                }

                return n * factorial(n - 1);
            });


            ExecuteFactorialExample(factorial);

            var sum = Memoize<int[], int>(array =>
            {
                Console.WriteLine($"Working for sum {string.Join(",", array)}");
                return array.Sum();
            });

            ExecuteSumExample(sum);
        }

        private static void ExecuteSumExample(Func<int[], int> sum)
        {
            Console.WriteLine(sum(new[] { 3 }));
            Console.WriteLine(sum(new[] { 4, 5 }));
            Console.WriteLine(sum(new[] { 3 }));
            Console.WriteLine(sum(new[] { 4, 5 }));
            Console.WriteLine(sum(new[] { 5, 6, 7 }));

            //Memoize

            //[3] not found in cache
            //Working for sum 3
            //3

            //[4,5] not found in cache
            //Working for sum 4,5
            //9

            //[3] found in cache
            //3

            //[4,5] found in cache
            //9

            //[5,6,7] not found in cache
            //Working for sum 5,6,7
            //18
        }

        private static void ExecuteFactorialExample(Func<int, int> factorial)
        {
            Console.WriteLine(factorial(3));
            Console.WriteLine(factorial(3));
            Console.WriteLine(factorial(4));
            Console.WriteLine(factorial(3));

            //Memoize

            //3 not found in cache
            //Working for factorial 3
            //2 not found in cache
            //Working for factorial 2
            //1 not found in cache
            //Working for factorial 1
            //6

            //3 found in cache
            //6

            //4 not found in cache
            //Working for factorial 4

            //3 found in cache
            //24

            //3 found in cache
            //6
        }

        public static Func<T, TResult> Memoize<T, TResult>(Func<T, TResult> fn)
        {
            Console.WriteLine(nameof(Memoize));
            var cache = new Dictionary<string, TResult>();
            return n =>
            {
                var key = JsonConvert.SerializeObject(n);
                if (cache.ContainsKey(key))
                {
                    Console.WriteLine($"{key} found in cache");
                    return cache[key];
                }
                Console.WriteLine($"{key} not found in cache");
                var value = fn(n);
                cache[key] = value;
                return value;
            };
        }
    }
}