WordCount.cs

using System;
using System.IO;
using System.Linq;

namespace WordCountCS
{
    class Program
    {
        static void Main(string[] args)
        {
            var sw = new System.Diagnostics.Stopwatch();
            sw.Start();
            Directory.EnumerateFiles(@"C:\TEMP\20_newsgroups", "*", SearchOption.AllDirectories)
                .AsParallel()
                .AsUnordered()
                .SelectMany(fn => File.ReadLines(fn))
                .SelectMany(line => line.Split(' ', '.', ':', ';', '?'))
                .GroupBy(key => key, element => element, (word, grouping) => new { word, count = grouping.Count() })
                .ForAll(x => Console.WriteLine("{0}:{1}", x.word, x.count));
            Console.Error.WriteLine("Elapsed: {0}", sw.Elapsed);
        }
    }
}