ASP.Net and Parallel.Foreach causes buttons to stop working?

Question

i have a very large database of images from the web which i am categorizing (downloaded locally).

so i have a website (locally) to do this, but the db queries were taking long, so i got an idea to "preload" the next page, so that only the very first load of the page would be slow. I save the list of items loaded in a seperate thread in session. So far so good. I wanted to optimize further, and did some testing on what took the longest, and loading the images to check the size to see if i needed to scale them (set image height and width on the img obj) - so i wanted to do this with a parallel.foreach loop - but after doing this, my buttons on the page stopped responding? i can see the page runs through the page_load event when i press a button, but it doesn't reach the buttons "code": protected virtual void btnSaveFollowPosts_Click(object sender, EventArgs e) {...}

any take on what i am doing wrong? i have tried to limit the degree of paralellelism to 1 just to see if that would fix it - but it did not.

Update - code: trying to boil it down:

protected void Page_Load(object sender, EventArgs e)
{
    Search(false);
}

protected void Search(bool updateCounters)
{
    if (Session[SessionItems] == null)
    {
         if (Session[SessionItemsCache] == null)
         {
             //if is being constructed, wait, else construct
             //if construction is not running
             if (Session[SessionCacheConstructionRunning] == null)
             {
                 StartPreLoadContent();
             }

             while (Session[SessionCacheConstructionRunning] != null)
             {
                Thread.Sleep(25); //block main thread untill items ready
             }
         }

         List<ContentView> contentViewList = Session[SessionItemsCache] as List<ContentView>;
         Session[SessionItemsCache] = null; //clean preload cache
         Session[SessionItems] = contentViewList; //save in current usage storage

         Filltable(ref tblContent, contentViewList);

         //preload next batch
         StartPreLoadContent();
    }
    else
    {
         List<ContentView> contentViewList = Session[SessionItems] as List<ContentView>; //get items from session
         Session[SessionItems] = contentViewList; //save in current usage storage

         Filltable(ref tblContent, contentViewList);
    }   
}

protected void StartPreLoadContent()
{
    Session[SessionCacheConstructionRunning] = true;

    //start task
    Thread obj = new Thread(new ThreadStart(RunPreLoadContent));
    obj.IsBackground = true;
    obj.Start();
}

protected void RunPreLoadContent()
{
    using (DBEntities entities = new DBEntities())
    {
        entities.CommandTimeout = 86400;

        IQueryable<ContentView> query = entities.ContentView.Where(some criterias);                            

        List<ContentView> contentViewListCache = query.ToList();

        ParallelOptions options = new ParallelOptions();
            options.MaxDegreeOfParallelism = 7;

            Parallel.ForEach(contentViewListCache, options, content =>
            {                   
                try
                {
                    Interlocked.Increment(ref imageSizeCount);

                    string path = Path.Combine(basePath, content.LocalPath);
                                    int imageSize = 150;

                    using (System.Drawing.Image realImage = System.Drawing.Image.FromFile(path))
                    {
                            double scale = 0;

                        if (realImage.Height > realImage.Width)
                        {
                            scale = (double)realImage.Height / imageSize;
                        }
                        else
                        {
                            scale = (double)realImage.Width / imageSize;
                        }

                        if (scale > 1)
                        {
                            content.ImageHeight = (int)((double)realImage.Height / scale);
                            content.ImageWidth = (int)((double)realImage.Width / scale);

                            content.ImageScaled = true;
                        }
                        content.ShowImage = true;
                    }
                }
                catch (Exception)
                {
                }
            });

        Session[SessionItemsCache] = contentViewListCache;
        Session[SessionCacheConstructionRunning] = null; //cache ready
    }

    protected virtual void btnSave_Click(object sender, EventArgs e)
        {
            try
            {
                //save
                ...some reading and saving going on here...

                //update
                Session[SessionItems] = null;
                Search(true);
            }
            catch (Exception error)
            {
                ShowError(error);
            }
        }

Answer 1

I agree with a previous comment: you should probably do this logic earlier in the page lifecycle. Consider overriding OnInit and putting it there.

Also, you could try this line of code instead of your current thread code (which is more suited to Windows not Web programming):

using System.Threading.Tasks;

Task.Run(() => { RunPreLoadContent(); });