Expression caching

class Program
    {
        // Can't do IDictionary<int, Func<T, String>> as T is not available here.
        private static readonly IDictionary<int, Object> _cache = new Dictionary<int, Object>();

        public static void SomeToStringCalls()
        {
            Console.WriteLine(ToString(i => (i + 1).ToString(), 1));
            Console.WriteLine(ToString(i => (i + 1).ToString(), 2));
            Console.WriteLine(ToString(i => (i + 2).ToString(), 3));
            Console.WriteLine(ToString(i => (i + 2).ToString(), 4));
        }

        private static int GetExpressionHash<T>(Expression<Func<T, string>> expression)
        {
            int hash = expression.Body.ToString().GetHashCode();
            // if you consider parameter unimportant - comment this line
            // using quite simple and error-prone hash function h = h * 31 + update
            // you might want to switch to md5 or sha in production
            expression.Parameters.ToList().ForEach(param => hash = hash * 31 + param.Name.GetHashCode());
            return hash;
        }

        private static string ToString<T>(Expression<Func<T, string>> expression, T input)
        {
            var cacheKey = GetExpressionHash(expression);
            if (!_cache.ContainsKey(cacheKey))
            {
                _cache[cacheKey] = expression.Compile();
                Console.WriteLine(String.Format("Compiling expression {0}", expression.ToString()));
            }
            var method = (Func<T, string>) _cache[cacheKey];
            return method.Invoke(input);
        }

        static void Main(string[] args)
        {
            SomeToStringCalls();
            Console.ReadLine();
        }
    }