MapReduce in C# using TPL.

MapReduce.cs

// http://ox.no/files/MapReduce.cs

public static class MapReduce
{
    public static Task<TResult> Start<TInput, TPartialResult, TResult>(Func<TInput, TPartialResult> map, Func<TPartialResult[], TResult> reduce, params TInput[] inputs)
    {
        var mapTasks = CreateMapTasks(map, inputs);

        var reduceTask = CreateReduceTask(reduce, mapTasks);

        return reduceTask;
    }

    private static Task<TResult> CreateReduceTask<TPartialResult, TResult>(Func<TPartialResult[], TResult> reduce, Task<TPartialResult>[] mapTasks)
    {
        return Task.Factory.ContinueWhenAll(mapTasks, tasks => PerformReduce(reduce, tasks));
    }

    private static TResult PerformReduce<TPartialResult, TResult>(Func<TPartialResult[], TResult> reduce, Task<TPartialResult>[] tasks)
    {
        var results = tasks.Select(task => task.Result);
        return reduce(results.ToArray());
    }

    private static Task<TPartialResult>[] CreateMapTasks<TInput, TPartialResult>(Func<TInput, TPartialResult> map, TInput[] inputs)
    {
        var tasks = new Task<TPartialResult>[inputs.Length];

        for (int i = 0; i < inputs.Length; ++i)
        {
            var input = inputs[i];
            tasks[i] = Task.Factory.StartNew(() => map(input));
        }

        return tasks;
    }
}