Fatal: Collection was modified; enumeration operation may not execute


To speed up our deployment process that have 68 steps we recently configured several of the steps to “Run in parallel with the previous step”. As a result we now often get a new error during deployment.

Looks like the Octopus code is not thread safe and it get’s a fatal error during deployment. The stack trace is always the same so it should hopefully be easy to find the problem.

                    |   == Failed: Step 58: TextAnalytics ==
17:36:54   Verbose  |     An operation in "TextAnalytics" failed
                    |     Collection was modified; enumeration operation may not execute.
                    |     System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
                    |     at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
                    |     at System.Collections.Generic.List`1.Enumerator.MoveNextRare()
                    |     at Octopus.Core.Model.Variables.VariableCollection.ToDictionary() in Y:\Work\refs\tags\3.4.4\source\Octopus.Core\Model\Variables\VariableCollection.cs:line 97
                    |     at Octopus.Server.Orchestration.Deploy.DeploymentPlan.CreateMachineCriteria(ICollection`1 roles) in Y:\Work\refs\tags\3.4.4\source\Octopus.Server\Orchestration\Deploy\DeploymentPlan.cs:line 177
                    |     at Octopus.Server.Orchestration.Deploy.DeploymentPlan.CreateMachineCriteria(PlannedAction action) in Y:\Work\refs\tags\3.4.4\source\Octopus.Server\Orchestration\Deploy\DeploymentPlan.cs:line 171
                    |     at Octopus.Server.Orchestration.Deploy.DeploymentTaskController.DistributeStepAcrossMachines(PlannedStep step) in Y:\Work\refs\tags\3.4.4\source\Octopus.Server\Orchestration\Deploy\DeploymentTaskController.cs:line 219
                    |     at Octopus.Server.Orchestration.Deploy.DeploymentTaskController.DispatchStep(PlannedStep step) in Y:\Work\refs\tags\3.4.4\source\Octopus.Server\Orchestration\Deploy\DeploymentTaskController.cs:line 103
                    |     Octopus.Server version 3.4.4 (3.4.4+Branch.master.Sha.ff35e6d80a68240faa886aa5940d18038c8bd45d)
17:36:54   Fatal    |     The step failed: Collection was modified; enumeration operation may not execute.
17:36:54   Verbose  |     TextAnalytics completed

I hope you will be able to provide a fix for this soon.


Hi Jan,

Thanks for getting in touch. Based on that stack trace we are already working on making this code deal better with concurrent actions: https://github.com/OctopusDeploy/Issues/issues/2719

Hope that helps!

That is good to hear :slight_smile:

I’ll monitor the issue. Hope you manage to get it resolved soon

All the best,