System.Diagnostics.Stopwatch -- always remeber to use Reset

Wrong

 
        System.Diagnostics.Stopwatch s = new System.Diagnostics.Stopwatch();

        s.Start();
        for (i = 0; i < count; i++) Test1();
        s.Stop();
        Console.WriteLine("Test1: {0}", s.ElapsedMilliseconds);

        s.Start();
        for (i = 0; i < count; i++) Test2();
        s.Stop();

        Console.WriteLine("Test2: {0}", s.ElapsedMilliseconds);

Right

 
        System.Diagnostics.Stopwatch s = new System.Diagnostics.Stopwatch();

        s.Start();
        for (i = 0; i < count; i++) Test1();
        s.Stop();
        Console.WriteLine("Test1: {0}", s.ElapsedMilliseconds);

        s.Reset();

        s.Start();
        for (i = 0; i < count; i++) Test2();
        s.Stop();

        Console.WriteLine("Test2: {0}", s.ElapsedMilliseconds);

Not that *cough* I would ever make this mistake or anything, but you know... it could happen to like, a friend, or something. Ya. My friend. :)

Comments

• Anonymous
  July 18, 2006
  Calling Reset() takes CPU time.  Your colleague's code ran faster without that call  ^_^
• Anonymous
  July 18, 2006
  The comment has been removed
• Anonymous
  July 18, 2006
  Yeah - and hopefully you catch it before the resulting #s make it into a spreadsheet, then a PPT, and then get presented to a bunch of people...  Not that I've made that mistake before either, mind you... ;)
• Anonymous
  July 18, 2006
  Yes but you were only doing that so I wouldn't look bad right?
  
  Err, what I mean is...  Hypothetically, if that had happened, then that would be hypothetically why it, uh, hypothetically, uh, happened.
  
  Ya.  That.
  
  grin
• Anonymous
  July 18, 2006
  Paraphrasing a famous performance architect, I suggest that you measure, measure, and measure … using three different methods to be able to cross check your results :-)
• Anonymous
  July 18, 2006
  You know the irony is I normally can't get my colleagues to read my blog without prodding.  I say something embarassing and the whole department rushes online :)
  
  You're all swines :)
• Anonymous
  July 19, 2006
  Call me lazy, but I always end up using the Startnew so I never forget to call reset :)
  
         Stopwatch s = Stopwatch.StartNew();
         for (i = 0; i < count; i++) Test1();
         s.Stop();
         Console.WriteLine("Test1: {0}", s.ElapsedMilliseconds);
  
         s = Stopwatch.StartNew();
         for (i = 0; i < count; i++) Test2();
         s.Stop();
  
         Console.WriteLine("Test2: {0}", s.ElapsedMilliseconds);
• Anonymous
  July 19, 2006
  Ya. I made the same mistake. I wish the class provides Pause !!
• Anonymous
  July 19, 2006
  People from the show business have known for ages that doing something embarrassing can increase your fan-base :-)
• Anonymous
  July 19, 2006
  GC, adding Pause might not help as much. Many people would never notice we added Pause. Something like "StartAgain" could, as it shows next to Start in documentation and the statement completion drop-down.
• Anonymous
  July 20, 2006
  Hi Krzysztof,
  
  My main point is to add a different function to do reset and start. Sometimes people like me assume what the function would do by looking at the function name :-) even though MSDN does state clearly:
  
  Starting a Stopwatch that is already running does not change the timer state or reset the elapsed time properties.
  
• Anonymous
  July 22, 2006
  Hi Rico,
  
  nice tip. I was always wondering how reliable this QueryPerformanceCounter thing is with Hyperthreaded, MulitCore or Multiprocessor machines. Imagine if I read the start counter on one processor and in the meantime I am scheduled to another processor. I will then? read the counter of another processor and get totally wrong times. Are there any guarantees that the counters of all cores/processors are synchronized?
  
  Yours,
   Alois Kraus