Thread-safe memoization

Question

Let\'s take Wes Dyer\'s approach to function memoization as the starting point:

public static Func Memoize(this Func f)
{         


        

Answer 1

Thomas's answer does not seem to compile under .NET 4.0 due to the enum parameter to the Lazy constructor. I revised it below. I also added an optional parameter for supplying one's own equality comparer. This is useful if TInput does not implement its own Equals or if TInput is a string and you want to make it case insensitive, for example.

    public static Func Memoize(
        this Func func, IEqualityComparer comparer = null)
    {
        var map = comparer == null
                      ? new ConcurrentDictionary>()
                      : new ConcurrentDictionary>(comparer);

        return input =>
               {
                   var lazy = new Lazy(() => func(input), LazyThreadSafetyMode.ExecutionAndPublication);

                   return map.TryAdd(input, lazy)
                              ? lazy.Value
                              : map[input].Value;
               };
    }

I did some basic testing of this method using this as my test:

    public void TestMemoize()
    {
        Func mainFunc = i =>
                                     {
                                         Console.WriteLine("Evaluating " + i);
                                         Thread.Sleep(1000);
                                         return i.ToString();
                                     };

        var memoized = mainFunc.Memoize();

        Parallel.ForEach(
            Enumerable.Range(0, 10),
            i => Parallel.ForEach(Enumerable.Range(0, 10), j => Console.WriteLine(memoized(i))));
    }

It seems to be working correctly.