I am using Linq2Umbraco to retrieve data. But sometimes I get an error saying:

"collection was modified enumeration operation"

and the only way to resolve the error is to restart the IIS.

The error appears in different places in my code, but always when retrieving something through Linq2Umbraco. 

I cache Linq2Umbraco for performance. In order to keep the cached LinqContext fresh,  I remove the context from the cache when publishing a document (see code below). I haven't been able to recreate the error, but it just happens sometimes one my server. At first I thought it might have something to do with publishing a node, which might cause the Linq2Umbraco-context to be out of sync, but I haven't been able to prove that. Any ideas why ?

thanks

Thomas

 public partial class LinqDataContext : IDisposable
    {
        private static object createContextLock = new object();
        public static LinqDataContext Instance
        {
            get
            {
                if (HttpContext.Current != null)
                {
                    var context = HttpContext.Current.Cache["LinqDataContext"] as LinqDataContext;
                    if (context == null)
                    {
                        lock (createContextLock)
                        {
                            if (context == null)
                            {
                                context = new LinqDataContext();
                                HttpContext.Current.Cache["LinqDataContext"] = context;
                                //to ensure the cache items are always fresh, we clear the context when changes are made to a document
                                Document.AfterNew += new EventHandler<umbraco.cms.businesslogic.NewEventArgs>(Document_AfterNew);
                                Document.AfterPublish += new Document.PublishEventHandler(Document_AfterPublish);
                                Document.AfterUnPublish += new Document.UnPublishEventHandler(Document_AfterUnPublish);
                                Document.AfterDelete += new Document.DeleteEventHandler(Document_AfterDelete);
                                Document.AfterMoveToTrash += new Document.MoveToTrashEventHandler(Document_AfterMoveToTrash);
                            }
                        }
                    }
                    return context;
                }
                else
                    return new LinqDataContext();
            }
        }
 
        public void Dispose()
        {
            Document.AfterNew -= new EventHandler<umbraco.cms.businesslogic.NewEventArgs>(Document_AfterNew);
            Document.AfterPublish -= new Document.PublishEventHandler(Document_AfterPublish);
            Document.AfterUnPublish -= new Document.UnPublishEventHandler(Document_AfterUnPublish);
            Document.AfterDelete -= new Document.DeleteEventHandler(Document_AfterDelete);
            Document.AfterMoveToTrash -= new Document.MoveToTrashEventHandler(Document_AfterMoveToTrash);
            base.Dispose();
        }
        public static void RefreshFromCache()
        {
            if (HttpContext.Current != null)
                HttpContext.Current.Cache.Remove("LinqDataContext");
        }
 
        static void Document_AfterMoveToTrash(Document sender, umbraco.cms.businesslogic.MoveToTrashEventArgs e)
        {
            RefreshFromCache();
        }
 
        static void Document_AfterDelete(Document sender, umbraco.cms.businesslogic.DeleteEventArgs e)
        {
            RefreshFromCache();
        }
 
        static void Document_AfterUnPublish(Document sender, umbraco.cms.businesslogic.UnPublishEventArgs e)
        {
            RefreshFromCache();
        }
 
        static void Document_AfterPublish(Document sender, umbraco.cms.businesslogic.PublishEventArgs e)
        {
            RefreshFromCache();
        }
 
        static void Document_AfterNew(object sender, umbraco.cms.businesslogic.NewEventArgs e)
        {
            RefreshFromCache();
        }
    }