Possible that Update() is multi-threaded?

Possible that Update() is multi-threaded?

• eshny

  Is it possible for you to upload the game somewhere If not, can you email it to me and I'll take a look
  
  

• db_guy

  BTW, you can see a screenshot of the debug details here:

  http://ilocker.bsu.edu/users/mstum/world_shared/xna_exception.gif

  
  

• Vaassu

  Hey Matt,
  
  maybe I'm wrong but I've had a similar strange behaviour like your problem some time ago.
  
  I did some collision detection stuff with a list of objects, I walked through the list with a
  foreach-statement and removed the objects that tested positive and had a very similar
  Error like you got.
  I can't remeber if the code was multithreaded or not, but I believe it wasn't.
  
  I fixed this problem by doing the foreach-loop and added the positive-tested objects to a
  temporal second list and used this list for removal ob the objects from the original list.
  This solved my problem and the error was gone.
  
  I guess the foreach-statement builds an iterator to walk through the list and if you remove
  an element the iterator gets corrupted because it's built at the beginning of the loop and
  then you get an error......
  
  Try it out, I think it works....let me know....
  
  
  Have fun.....
  
  

• JustStudent

  Found this recently.....
  
  very neat solution :-)
  
  thx .net
  
  
  
  
  

• James Strickland

  Don't forget that you can post game projects up on www.codeplex.com!

  
  
  

• Peter Feigl

  Very nice. I'm filing that piece of code away for future reference.
  
  

• aschaeffer

  Shellacc wrote:

  Hey Matt, I guess the foreach-statement builds an iterator to walk through the list and if you remove an element the iterator gets corrupted because it's built at the beginning of the loop and then you get an error......

  
  
  Exactly. The CLR does not allow you to modify a collection at the same time as you are iterating over it, so you can't add or remove elements from inside a foreach loop.
  
  
  

• roy-roy

  I *may* have found the problem. Further down in the Particle.Update routine I found a bad piece of logic that could exceed a Color[] array index. I've been running this code off and on for the better part of the week with a constant 600+ particles at all times without any problems... the logic error is based on such a rare timing issue that it just never surfaced. However, when I ran it via Ctrl-F5 instead of F5 the FPS went from 30 to 200. I think it took the smaller time slices to bring out the problem (even though it was still hard to replicate).

  I've fixed the logic error and I haven't seen the exception yet, although I'm not ruling anything out. It's just odd that the stack dump is so misleading.

  For now I'll consider the case closed and let you guys get on to more important things. If the problem resurfaces I'll give this thread a bump and publish the entire source package.

  
  

• Santhoo

  Stupid forum errors! You can email it to me at machaira-at-comcast-dot-net.
  
  

• Sandeep A

  I ran into the same thing, but this is quite a different problem. If you look at my earlier code snippits above you'll see where I iterate through each of the particles and then call the following line:

  if (p.delete) dead.Add(p);

  I then rip through all of the items in "dead" and remove them from "particles". I then call p.Dispose() on each of the particles. Once I'm done I call dead.Clear();

  But no, that problem shows up each and every time... easy to replicate. What I ran across was a stack dump that wasn't really telling me where the offending line was... that made it a LOT harder to track down the real problem (assuming I've fixed it).

  
  

• BilalShouman

  Here's the snippet in my PlayComponent that is ripping through all of the particles:

  foreach (Particle p in particles) {
  p.Update(elapsed);
  if (p.delete) dead.Add(p);
  }

  particles and dead are both List<Particle>

  The line that actually throws the exception is in Particle.Update. Particle is derived from Sprite.

  public override void Update(float elapsed) {
  base.Update(elapsed); // <=== This is the line that throws the exception
  age += elapsed;
  // ... etc ...

  And just for reference, here's the entire contents of Sprite.Update:

  public virtual void Update(float elapsed) {
  if ((acceleration.X != 0.0F)||(acceleration.Y != 0.0F)) {
  velocity += Vector2.Multiply(acceleration, elapsed);
  }
  if ((velocity.X != 0.0F) || (velocity.Y != 0.0F)) {
  position += Vector2.Multiply(velocity, elapsed);
  }
  if (roationspeed != 0.0F) {
  rotation = (rotation + ((roationspeed * MathHelper.TwoPi) * (elapsed / 60.0F))) % MathHelper.TwoPi;
  }
  }

  
  

• wva

  Very mysterious! That code all looks fine to me.
  
  Hopefully someone else will come along and spot the obvious error that we're both missing, and make us look stupid :-)
  
  
  

• MicahN

  The CLR doesn't do any magic threading behind your back, so that can't be the problem.
  
  It would be useful if you could post the source for the method that is actually throwing the exception (First.Particle.Update(Single elasped)).
  
  
  

• ZopoStyle

  The same solution is used in SpaceWar for removing bullets and particles etc from the scenegraph. See SceneItem.cs, I guess its not all bad design after all :-)
  
  
• Possible that Update() is multi-threaded?