An item with the same key has already been added

Question

I am getting this error randomly at the production server here is the stack trace of the error. I am using linq to sql and .net 4.0

System.ArgumentException: An item with the same key has already been added.
   at System.ThrowHelper.ThrowArgumentException(ExceptionResource resource)
   at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
   at System.Data.Linq.DataContext.GetTable(MetaTable metaTable)
   at System.Data.Linq.DataContext.GetTable[TEntity]()
   at UserManagement.Models.EvoletDataContext.get_sysModules() in D:\MyProj\Models\datamdl.designer.cs:line 1294
   at UserManagement.Models.FilterRepository.GetModuleHead(String actionName) in D:\MyProj\Models\FilterRepository.cs:line 14
   at UserManagement.Models.DummyAttrib.OnAuthorization(AuthorizationContext filterContext) in D:\MyProj\Models\Filters.cs:line 44
   at Glimpse.Net.Plumbing.GlimpseAuthorizationFilter.OnAuthorization(AuthorizationContext filterContext)
   at System.Web.Mvc.ControllerActionInvoker.InvokeAuthorizationFilters(ControllerContext controllerContext, IList`1 filters, ActionDescriptor actionDescriptor)
   at System.Web.Mvc.ControllerActionInvoker.InvokeAction(ControllerContext controllerContext, String actionName)
   at System.Web.Mvc.Controller.ExecuteCore()
   at System.Web.Mvc.ControllerBase.Execute(RequestContext requestContext)
   at System.Web.Mvc.ControllerBase.System.Web.Mvc.IController.Execute(RequestContext requestContext)
   at System.Web.Mvc.MvcHandler.<>c__DisplayClass6.<>c__DisplayClassb.<BeginProcessRequest>b__5()
   at System.Web.Mvc.Async.AsyncResultWrapper.<>c__DisplayClass1.<MakeVoidDelegate>b__0()
   at System.Web.Mvc.Async.AsyncResultWrapper.<>c__DisplayClass8`1.<BeginSynchronous>b__7(IAsyncResult _)
   at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResult`1.End()
   at System.Web.Mvc.MvcHandler.<>c__DisplayClasse.<EndProcessRequest>b__d()
   at System.Web.Mvc.SecurityUtil.<GetCallInAppTrustThunk>b__0(Action f)
   at System.Web.Mvc.SecurityUtil.ProcessInApplicationTrust(Action action)
   at System.Web.Mvc.MvcHandler.EndProcessRequest(IAsyncResult asyncResult)
   at System.Web.Mvc.MvcHandler.System.Web.IHttpAsyncHandler.EndProcessRequest(IAsyncResult result)
   at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

Code at line 14 is below. I have also included the datacontext

private EvoletDataContext db = new EvoletDataContext();

        public sysModule GetModuleHead(string actionName)
        {
            var val =  (from mod in db.sysModules
                        where
                        mod.ModuleActionResult.ToLower().Equals(actionName.ToLowerInvariant())
                    select mod).SingleOrDefault();
            return val;
    }

Code at line 44 is

public class DummyAttrib:FilterAttribute,IAuthorizationFilter
    {

        private readonly FilterRepository _filterRepository = new FilterRepository();
        public void OnAuthorization(AuthorizationContext filterContext) 
        {
            if (!filterContext.Controller.ControllerContext.IsChildAction && !filterContext.HttpContext.Request.IsAjaxRequest())
            {
                var cont = (ApplicationController)filterContext.Controller;
                                var modhead = _filterRepository.GetModuleHead(filterContext.RouteData.Values["action"].ToString());
                if (cont.DocumentID != 0 && modhead !=null) 
                {
                    if (_filterRepository.hasRightonModuleChildren(modhead.ModuleID, cont.RoleID))
                        return;
                }
                if (cont.DocumentID == 0 && !filterContext.RouteData.Values["action"].ToString().ToLowerInvariant().Equals("index"))
                {
                    filterContext.Result = new RedirectResult("/account.mvc/AccessDenied");
                    return;
                }

                if (!_filterRepository.hasRighton(cont.DocumentID, cont.RoleID))
                {
                    filterContext.Result = new RedirectResult("/account.mvc/AccessDenied");
                    return;
                }

            }
        }
    }

Answer 1

LINQ maintains a 'cache' of sorts of tables you have already used, to prevent having to look them up multiple times.

at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
at System.Data.Linq.DataContext.GetTable(MetaTable metaTable)
at System.Data.Linq.DataContext.GetTable[TEntity]()

The table you're trying to use hasn't been used within the data context before, so it's being added to the 'cache'. One of the ways this could fail is if you're calling GetTable() from multiple threads simultaneously, which suggests you're using a single data context for the entire application, which is not a good idea. Also, don't use the same data context if you're executing tasks in parallel.