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I am getting a tricky error on an MVC page that only occurs when multiple users are on the site.

The error I get is

Collection was modified; enumeration operation may not execute.

The error occurs on the line that attempts to render a javascript file Bundle.

 @Scripts.Render("~/Scripts/js")

The associated code in /app_start -> BundleConfig.cs -> RegisterBundles is pretty standard:

bundles.Add(new ScriptBundle("~/Scripts/js").Include(
            "~/Scripts/jquery-{version}.js",
            "~/Scripts/jquery-ui-{version}.js",
            "~/Scripts/jquery.validate*",
            "~/Scripts/jQuery.Buzz/jquery.buzz.js"
        ));

The error only seems to occur when there are multiple users on the site at once. I managed to reproduce the problem locally by creating multiple tabs in multiple browsers and hitting them as quick as I can.

I've searched and haven't been able to find anyone else with a similar error. The error implies that the a collection is being modified while being looped over. I can't see how the collection of files to be added to the bundle could be changing as the collection is being enumerated.

The Stack trace is as follows:

[InvalidOperationException: Collection was modified; enumeration operation may not execute.]
   System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource) +56
   System.Collections.Generic.Enumerator.MoveNextRare() +14277518
   System.Web.Optimization.Bundle.EnumerateFiles(BundleContext context) +266
   System.Web.Optimization.Bundle.GenerateBundleResponse(BundleContext context) +38
   System.Web.Optimization.Bundle.GetBundleResponse(BundleContext context) +61
   System.Web.Optimization.BundleResolver.GetBundleContents(String virtualPath) +214
   System.Web.Optimization.AssetManager.DeterminePathsToRender(IEnumerable`1 assets) +410
   System.Web.Optimization.AssetManager.RenderExplicit(String tagFormat, String[] paths) +35
   ASP._Page_Views_Shared__Layout_cshtml.Execute() in c:\..\Views\Shared\_Layout.cshtml:13

The server was not particularly overloaded at the time the errors occurred. I have monitoring software on the server that showed the CPU and memory at normal levels while the errors were generated.

Any ideas what could be causing this?

UPDATE: After some further searching I discovered that the Bundle was being updated programmatically in the Controller.

  BundleTable.Bundles.GetBundleFor("~/Scripts/js").Include("~/Scripts/jquery.mobile-{version}.js");

It turns out this didn't need to be added programmatically so I have included it in the BundleConfig file and the site seems to be behaving now.

KiwiPyc
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  • The `Scripts.Render()` function shouldn't cause such exception. Post the content of the BundleConfig.cs file to see if there is anything wrong there, specifically the `~/Scripts/js` path. – Racil Hilan Jun 02 '16 at 05:47
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    the error you got happens if you were using foreach() statement on a collection and collection changes within the execution of foreach() . – Amir Shrestha Jun 02 '16 at 08:08
  • Thanks for the comments. I am about to try replacing the {version} wildcard in BundleConfig.cs with explicit versions to see if that makes any difference. – KiwiPyc Jun 02 '16 at 21:55
  • Getting rid of all the wildcards from the BundleConfig.cs didn't fix the issue. It seemed like it improved things but further testing showed the problem is still there. – KiwiPyc Jun 03 '16 at 02:01

1 Answers1

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try adding this to your bundle config file under app_start 

public static void RegisterBundles(BundleCollection bundles) {
        bundles.Add(new ScriptBundle("~/bundles/js1").Include("~/js/jquery-1.8.3.min.js","~/js/jquery-ui.js"));
    }

the "~/bundles/js1" is the name of the bundle.

in your view add this line tor render the scripts into the view.

@System.Web.Optimization.Scripts.Render("~/bundles/js1")

that should work fine... just don't forget to change the included js file name and the name of the bundle

RoyM
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  • Thanks Roy. That's what I've already done. I've added some more details to my question to clarify. – KiwiPyc Jun 02 '16 at 22:02