"Collection was modified; enumeration operation may not execute"

[td_910]

Once in a while I get "Collection was modified; enumeration operation may not execute" with that piece of code I think.
It should read out the levels from a user drawn fibonacci retracement and add them to a list.

            if (IsFirstTickOfBar && DrawObjects["MyFib"] as DrawingTools.FibonacciRetracements != null)
            {
                dynamic temp = DrawObjects["MyFib"];
                if (temp.ToString().Equals("NinjaTrader.NinjaScript.DrawingTools.FibonacciRetracements"))
                {
                    foreach (PriceLevel p in temp.PriceLevels)
                    {
                        if (Time[0]>temp.EndAnchor.Time && !(levels.ContainsKey("FibLevel"+p.Value.ToString(""))))
                            levels.Add("FibLevel"+p.Value.ToString(""),p.GetPrice(temp.StartAnchor.Price, temp.EndAnchor.Price - temp.StartAnchor.Price, true));
                    }
                }
            }

Any ideas how can I change the code to prevent that?

Many thanks

[NinjaTrader_Emily]

Hello td_910,

Thank you for your post.

I believe the typical cause of this message has to do with attempting to modify a collection while it is being looped through. Please add Print() statements inside of each condition from this code snippet to further narrow down the offending line of code. For more details on using prints for debugging:

https://ninjatrader.com/support/forum/forum/ninjatrader-7/strategy-development-aa/99209-date-conditions-not-working#post791121

Are there particular steps you are taking that result in this error being thrown? For example, does it happen when manually modifying the user-drawn fib object? It is typically better to loop through the DrawObjects collection in a thread-safe way as demonstrated in the snippet at the bottom of this page:

https://ninjatrader.com/support/helpGuides/nt8/drawingtools_drawobjects.htm

  // Loops through the DrawObjects collection via a threadsafe list copy
  foreach (DrawingTool draw in DrawObjects.ToList())​

Even working with DrawObjects.ToList() could likely prevent this error from happening.

Please let us know if we may be of further assistance.

[bltdavid]

Any ideas how can I change the code to prevent that?

Maybe you could lock the collection?

An example is here.

It's possible something else is modifying the collection,
after all, NinjaTrader is multi-threaded -- thus the above
example locks Account.Positions even for a simple loop
that does nothing but print the positions.

If feasible¹, I'd tend to suggest lock over ToList.

Presumably, ToList does lots of extra work in making
an independent copy of the collection.

Just my 2˘.



1. Remember, lock is a cooperative thread thing and
requires the internal NT code to participate in using it.
That is, Internally, NinjaTrader code may (or may not)
be using lock for the DrawObjects, so even though
you are using it (which prevents other instances of your
indicator from entering the loop in their instance until
your instance is finished) it's possible to still have
problems if other internal parts of NT do not use the
same lock. That may be why NT help guide suggests
the ToList approach -- because under the hood the
NT engrs did not use lock when adding/removing items
from the DrawObjects collection.

[td_910]

Hello td_910,

Thank you for your post.

I believe the typical cause of this message has to do with attempting to modify a collection while it is being looped through. Please add Print() statements inside of each condition from this code snippet to further narrow down the offending line of code. For more details on using prints for debugging:
https://ninjatrader.com/support/foru...ing#post791121

Are there particular steps you are taking that result in this error being thrown? For example, does it happen when manually modifying the user-drawn fib object? It is typically better to loop through the DrawObjects collection in a thread-safe way as demonstrated in the snippet at the bottom of this page:
https://ninjatrader.com/support/help...rawobjects.htm

 // Loops through the DrawObjects collection via a threadsafe list copy
foreach (DrawingTool draw in DrawObjects.ToList())​

Even working with DrawObjects.ToList() could likely prevent this error from happening.

Please let us know if we may be of further assistance.

Thanks for your reply.
Since I know the tag of the draw object and I'm sure it's properties aren't modified by the user or another indicator, can't I just use one of these three methods:

protected override void OnBarUpdate()
{
  if (DrawObjects["someTag"] != null && DrawObjects["someTag"] is DrawingTools.Line)
  {
    // Do something with the drawing tool line
  }        

  // An alternative approach to find the draw object by a tag
  if (DrawObjects["someTag"] as DrawingTools.Line != null)
  {
    // Do something drawing tool line
  }  

  // Yet another way to find a drawing tool by a tag
  if (DrawObjects["someTag"].GetType().Name == "Line")
  {
    // Do something drawing tool line  
  }
}​

[NinjaTrader_ChelseaB]

Hello td_910,

The issue is with looping through the collection.

As long as you are using the specific tag and not looping through the collection, this would be fine to do.