C# Memmoizer

example.cs

namespace Memoizer
{

    class Example
    {
        Memoizer Memoizer = new Memoizer();

        bool AlwaysTrue()
        {
            return Memoizer.Memoize("AlwaysTrue", () => true);
        }

        ExpensiveObject GetFromGithub(string hash, int time)
        {
            return Memoizer.Memoize("GetFromFacebook", () => (new GitHubber()).ReallyExpensiveMethod() );
        }
    }
}

Memoizer.cs

namespace Memoizer
{
    /// <summary>
    /// Store a value so you don't run it twice
    /// </summary>
    public class Memoizer
    {
        private IDictionary<string, object> _frozenValues = new Dictionary<string, object>();

        /// <summary>
        /// Store a value so you don't run it twice
        /// </summary>
        /// <typeparam name="U">The type returned by the function</typeparam>
        /// <param name="name">The name to be persisted. Can't already exist.</param>
        /// <param name="lambda">The function to be used.</param>
        /// <returns>The value of the lambda.</returns>
        public U Memoize<U>(string name, Func<U> lambda = null)
        {
            if (!_frozenValues.ContainsKey(name))
            {
                if (lambda == null)
                    return default(U);

                _frozenValues.Add(name, lambda.Invoke());
            }
            return (U)_frozenValues[name];
        }

    }
}