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Editorial Team
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Asked: 2026-06-01T06:41:39+00:00

we’ve been getting this error lately and have no idea why, we are not

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we’ve been getting this error lately and have no idea why, we are not modifying any collection, we even removed most foreach on the pages that we were getting the error. Here is the stacktrace of the error:

   Exception type: InvalidOperationException 
    Exception message: Collection was modified; enumeration operation may not execute.
   en System.Collections.Generic.List`1.Enumerator.MoveNextRare()
   en System.Linq.Enumerable.WhereListIterator`1.MoveNext()
   en System.Linq.Enumerable.<SelectManyIterator>d__14`2.MoveNext()
   en System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
   en System.Linq.Enumerable.<SelectManyIterator>d__14`2.MoveNext()
   en System.Linq.Buffer`1..ctor(IEnumerable`1 source)
   en System.Linq.OrderedEnumerable`1.<GetEnumerator>d__0.MoveNext()
   en System.Linq.Buffer`1..ctor(IEnumerable`1 source)
   en System.Linq.Enumerable.<ReverseIterator>d__a0`1.MoveNext()
   en System.Web.Mvc.FilterProviderCollection.<RemoveDuplicates>d__b.MoveNext()
   en System.Linq.Buffer`1..ctor(IEnumerable`1 source)
   en System.Linq.Enumerable.<ReverseIterator>d__a0`1.MoveNext()
   en System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
   en System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   en System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   en System.Web.Mvc.FilterInfo..ctor(IEnumerable`1 filters)
   en System.Web.Mvc.ControllerActionInvoker.GetFilters(ControllerContext controllerContext, ActionDescriptor actionDescriptor)
   en System.Web.Mvc.ControllerActionInvoker.InvokeAction(ControllerContext controllerContext, String actionName)
   en System.Web.Mvc.Controller.ExecuteCore()
   en System.Web.Mvc.ControllerBase.Execute(RequestContext requestContext)
   en System.Web.Mvc.MvcHandler.<>c__DisplayClass6.<>c__DisplayClassb.<BeginProcessRequest>b__5()
   en System.Web.Mvc.Async.AsyncResultWrapper.<>c__DisplayClass1.<MakeVoidDelegate>b__0()
   en System.Web.Mvc.MvcHandler.<>c__DisplayClasse.<EndProcessRequest>b__d()
   en System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   en System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

Any help is appreciated.

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1 Answer

  1. Editorial Team

    More code of your list instantiation would help.

    Realistically, I would check that the enumerable that you pass to the list constructor is not changed. When you use IEnumerable, you are generating the enumerable at that point, whereas IQueryable allows you to defer the generation of the enumerable until a control point of execution.

    To me, it looks like you are generating an IEnumerable To pass to the list constructor, but you are modifying the source list before you do.

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