Collection was modified

[NinjaTrader_ChrisL]

Hello td_910,

Thanks for your reply.

The easiest way to find the error would be to use Visual Studio and have the debugger break when the exception happens.

See this video I made earlier on how to set up NinjaTrader for Visual Studio debugging:

https://clauber.tinytake.com/sf/MzQyODI5OF8xMDI2MjQ2NQ

Here is the help guide page on the subject as well:

https://ninjatrader.com/support/helpGuides/nt8/en-us/?visual_studio_debugging.htm

Run the script after attaching visual studio to the NinjaTrader.exe process and wait for the exception to occur. You should see Visual Studio stop at the line where the exception occurs.

Please let me know if I can assist any further.

[td_910]

thanks, I'll do that

one general question: Can I try to remove an object that doesn't exist without getting an exception?

e.g. RemoveDrawObject("Long_B1_TickFailure"+unique+(Cur rentBar - 1));
without the object being existent?

[NinjaTrader_ChrisL]

Hello td_910,

Thanks for your reply.

After testing with an indicator, I found that if you call RemoveDrawObject on a tag that does not exist, the action will be ignored and code flow will continue. We will need to find the exact line where the exception is thrown using Visual Studio debugging.

[td_910]

thanks, that's what I thought too

yes, I'll do that

can I debug more than one indicator at a time?

[td_910]

switching work spaces I got this: https://www.screencast.com/t/w81yYNs8R
has probably nothing to do with my indicators?

VS is running the whole night, but that error seems to be elusive

[NinjaTrader_ChrisL]

Hello td_910,

Thanks for your reply.

The error from your screenshot is not related to the script. There is something happening within either the workspace you switched from or to. You can restore a previous version of either workspace by going to Control Center>Database Management> Expand "Restore Workspace".

You can also debug two scripts at once. If either of them throw an exception, Visual Studio will break at the exception.

Please let me know if I can assist further.

[td_910]

Chris,

thanks for bringing up VS to debug NinjaScript. I wonder how I could do without that. :-)
found a copy/paste error, so far

[td_910]

here is another one, with the exact line of code
I'm just checking on an object, if that exists or not, no foreach or casting

https://www.screencast.com/t/v4T8wxEjt

[td_910]

I didn't expect this line to throw an exception

[NinjaTrader_ChrisL]

Hello td_910,

Thanks for your patience.

It looks like the help guide example where they access the DrawObjects collection with a tag name could cause this error, so if you could Export your script as source and send in your script to platformsupport[at]ninjatrader[dot]com and reference the forum thread link in the body of the email. I will reproduce and report the problem to our product management team so this can be fixed.

As a solution for you now, please use the "Looping through the collection to find specific draw objects" example on the DrawObjects help guide page.

https://ninjatrader.com/support/help...rawobjects.htm

After implementing the script in this way, you should not see the collection modified problem.

I look forward to assisting further.

[td_910]

the script has 1000+ lines of proprietary source code, which I don't wanna give away

dealing with chart objects in NT8 seems to become a night mare, compared to NT7
especially, if I need to use a thread safe list copy and do the dynamic cast for every single object

if I just want to check whether it exists or not

I don't hope, a condition like that would throw this error too

            #region Alert
            if (IsFirstTickOfBar && showAlert)
            {
                RearmAlert("PABuySellPressure_BGP");
                RearmAlert("PABuySellPressure_SGP");

                //BGP
                if (DrawObjects["BSP_Bull_Larger_Bodies"+(CurrentBar-1)] != null ||
                    DrawObjects["BSP_Bear_Smaller_Bodies"+(CurrentBar-1)] != null ||

                    DrawObjects["BSP_Bull_UTail_Decreasing"+(CurrentBar-1)] != null ||
                    DrawObjects["BSP_Bull_LTail_Decreasing"+(CurrentBar-1)] != null ||
                    DrawObjects["BSP_Bear_LTail_Increasing"+(CurrentBar-1)] != null ||
                    DrawObjects["BSP_Bear_UTail_Increasing"+(CurrentBar-1)] != null ||

                    DrawObjects["BSP_Bear_Overlap_Increasing"+(CurrentBar-1)] != null ||
                    DrawObjects["BSP_Bull_Overlap_Decreasing"+(CurrentBar-1)] != null ||

                    DrawObjects["BSP_BrsOnOpen"+(CurrentBar-0)] != null
                    )
                    Alert("PABuySellPressure_BGP", Priority.High, "PABuySellPressure - Buying Pressure", SoundFileBuy, 1, Alert_Background, bull_Symbol_Color);

I have several pieces of code, that are similar to that and they work without throwing exceptions

            if (DrawObjects["RVB_Sell_Text"+(CurrentBar-1)] != null && Low[1]<=Low[0] && Open[0]<=Close[0] && (Median[0]<=Close[0] || High[1]<High[0]))
            {
                dynamic RVB_Text = DrawObjects["RVB_Sell_Text"+(CurrentBar-1)];
                if (RVB_Text.ToString().Equals("NinjaTrader.NinjaScript.DrawingTools.Text"))
                    if (RVB_Text != null) RVB_Text.DisplayText = "(R)";
            }

[NinjaTrader_ChrisL]

Hello td_910,

Thanks for your reply.

The most simple refactor of this code you could do is create a method that takes a string (the draw objects tag), performs the FOR loop search, then returns the draw object or true if it exists, then replace your null checks with that method call. I'll create a script that attempts to reproduce this. If it is unsuccessful in re-creating the issue I will need you to make a stripped-down example that re-creates the same problem, we can submit that script to our development team so this workaround can be avoided. Also please let me know when this happens (during run time, a chart refresh, NinjaScript refresh)

I look forward to assisting further.

[td_910]

I think I remember, that the issue occurred either during a chart refresh or NinjaScript refresh, not during run time, because the error occurred on the 1st bar, the script processed

I use tons of null checks in my code, because it is based on chart objects entirely

I really don't get the point, why these null checks don't work (reliably) anymore in NT8 or why they throw the occasional exception

if multi-threading is the problem, why isn't the null check not thread safe by default and what is a not thread safe null check for

[td_910]

here you go: https://www.screencast.com/t/ZIRh1dh3
reload all historical data, while there are three other indicators loaded

the errors seem to be intermittent, not only the 1st bar, here bar 169
and another error as well ("Object reference not set to an instance of an object")

the 1st two runs were my proprietary indicator (indicator name was empty)

https://www.screencast.com/t/IH3hK7ZFQR2o

this probably happens, because a price change is processed already by the indicator, while the other three indicators still are modifying the draw objects collection

"what is a not thread safe null check for"

[NinjaTrader_ChrisL]

Hello td_910,

Thanks for posting a test script. Ill test it out and let you know what I find.

Thanks in advance for your patience.