Can't remove item from list in XNA

Question

I'm working on a Flappy Bird clone as an exercise, for I've recently started programming on XNA and I'm stuck on this error that I cannot understand.

I included the following code inside the Update() function, which's job is to delete the pipes when they go off screen lest they infinitely move leftwards while more are created:

//Pipe despawner
foreach (var pipe in pipes)
{
   if (pipe.position1.X <= -180)
   {
         pipes.Remove(pipe);
   }
}

The game runs fine until the first pipe goes offscreen and the debugger pauses and signals this part of the code with the following message:

An unhandled exception of type 'System.InvalidOperationException' occurred in mscorlib.dll

Additional information: Colección modificada; puede que no se ejecute la operación de enumeración.

I'm sorry the second part is in spanish, it's my system's language, but hopefully you know how to solve the problem anyway.

I believe I could simply not include this part of the code, given the simplicity of the game and, by consequence, the small toll that infinitely spawning pipes take on performance, but I'd rather not adopt this practice as soon as I start learning game programming.

Answer 1

The problem is that you are modifying the collection (by removing a pipe) while enumerating the collection. This will always throw an exception in .NET (and is something to be mindful of if you ever do this in a multi-threaded environment, since you can get some nastily non-reproducible exceptions because of it).

An easy way to solve it is to just keep a "kill" list while enumerating:

List killList = new List();

foreach (var pipe in pipes)
{
   if (pipe.position1.X <= -180)
   {
      killList.Add(pipe)
   }
}

Even easier:

IEnumerable<Pipe> killList = pipes.Where (p => p.position1.X < -100);

Either way, enumerate over this new collection, and remove matching elements from the main one (thus avoiding the error condition):

foreach (Pipe p in killList)
   pipes.Remove(p);

And you are done! Again, be mindful of threads. If you are creating new pipes in another thread, you could easily "collide" with this one and cause the exception. If that is the case, make sure to put locks around these sections of code.

Note, you could actually inline "killList" if you use the LINQ method:

foreach (Pipe p in pipes.Where(p => p.Position1.X <= -100))
   pipes.Remove(p);

As @rot13 suggested, you could also just run RemoveAll, passing it the predicate from the "Where" statement like:

pipes.RemoveAll(p => p.Position1.X <= -100);

That's about as simple as it gets :)

Answer 2

You can't remove an item from the collection you're currently iterating on with foreach.

Either use a for loop and adjust the indexes yourself, or use a second collection to handle the pipes you want to delete :

List<Pipe> piesToRemove = new List<Pipe>();
// First you flag every pipes you need to remove
foreach (var pipe in pipes)
{
   if (pipe.position1.X <= -180)
   {
         pipesToRemove.Add(pipe);
   }
}

//Now you remove them from your original collection
foreach (var pipe in pipesToRemove)
{
    pipes.Remove(pipe);
}