DisposableStopwatch.cs

public class DisposableStopwatch : Stopwatch, IDisposable
{
    public delegate string OnStopDelegate(TimeSpan time);
    private readonly OnStopDelegate _onStop;

    public DisposableStopwatch(OnStopDelegate onStop)
    {
        _onStop = onStop;
        Start();
    }

    public void Dispose()
    {
        Stop();
        Debug.WriteLine(_onStop(Elapsed));
    }
}