C#

var MouseDown = Observable.FromEventPattern<MouseButtonEventArgs>(this, "MouseDown");
var MouseUp = Observable.FromEventPattern<MouseButtonEventArgs>(this, "MouseUp");

var LeftMouseButtonPressedDuration = Observable.Join(
                  MouseDown
                  .Where(evt => evt.EventArgs.ChangedButton == MouseButton.Left)
                  .Select(_ =>
                  {
                      DateTime now = DateTime.Now;
                      return now;
                  }
                  ),
                  MouseUp
                   .Where(evt => evt.EventArgs.ChangedButton == MouseButton.Left)
                   .Select(_ =>
                    {
                        DateTime now = DateTime.Now;
                        return now;
                    }
                    ),

                  MouseDownTimeStamp => MouseDown,
                  MouseUpTimeStamp => Observable.Empty<DateTime>(),

                  (TimeDown, TimeUp) =>
                  {
                      TimeSpan MousePressedDuration = TimeUp - TimeDown;
                      return " Total duartion is: " + MousePressedDuration.ToString(@"ss\:fff");
                  }
                );

LeftMouseButtonPressedDuration.Subscribe(evt =>
{
    txtMainWindow.Text += Environment.NewLine + evt.ToString();
});

MouseUpTimeStamp => Observable.Empty<DateTime>()

MouseDownTimeStamp => MouseDown.Publish(),
 MouseUpTimeStamp => Observable.Empty<DateTime>(),

var Interval_50ms = Observable.Interval(TimeSpan.FromMilliseconds(50))
    .Take(10);

var Interval_100ms = Observable.Interval(TimeSpan.FromMilliseconds(100))
    .Take(5)
    .Select(i => Convert.ToChar(i + 65).ToString().ToUpper());

var tt = Observable.Join(
    Interval_50ms,
    Interval_100ms,
    s => Interval_50ms,
    c => Observable.Empty<char>(),
    (s, c) => { return s.ToString() + ", " + c.ToString(); })
    .Subscribe(Console.WriteLine);

3, A
3, A
3, B
6, D
8, E

0, A
2, B
4, D
6, E

public static string Decrypt(String text)
      {
          SymmetricAlgorithm algorithem = DES.Create();
          ICryptoTransform transform = algorithem.CreateEncryptor(key, iv);
          byte[] inputbuffer = Convert.FromBase64String(text);
          byte[] outputbuffer = transform.TransformFinalBlock(inputbuffer, 0, inputbuffer.Length);

          return Encoding.Unicode.GetString(outputbuffer);

      }