gistfile1.txt

  class Program
  {
      static ConcurrentDictionary<string, string> dict = new ConcurrentDictionary<string, string>();

      static void Main(string[] args)
      {
          var t1 = new Thread(() =>
          {
              dict.GetOrAdd("key1", x =>
                  {
                      Console.WriteLine("{1} GetOrAdd {0}", x, DateTime.Now);
                      Thread.Sleep(TimeSpan.FromSeconds(10));
                      return x;
                  });
          });

          var t2 = new Thread(() =>
          {
              dict.GetOrAdd("key2", x =>
                  {
                      Console.WriteLine("{1} GetOrAdd {0}", x, DateTime.Now);
                      return x;
                  });
          });

          var t3 = new Thread(() =>
          {
              dict.GetOrAdd("key1", x =>
              {
                  Console.WriteLine("{1} GetOrAdd {0}", x, DateTime.Now);
                  return x;
              });
          });

          t1.Start();
          Thread.Sleep(500);
          t2.Start();
          t3.Start();

          t1.Join();
          t2.Join();
          t3.Join();
      }
  }