Async task ASP.net HttpContext.Current.Items is empty - How do handle this?

Question

We are running a very large web application in asp.net MVC .NET 4.0. Recently we had an audit done and the performance team says that there were a lot of null reference exceptions.

So I started investigating it from the dumps and event viewer. My understanding was as follows:

We are using Asyn Tasks in our controllers. We rely on HttpContext.Current.Items hashtable to store a lot of Application level values.

Task<Articles>.Factory.StartNew(() =>
        {
            System.Web.HttpContext.Current = ControllerContext.HttpContext.ApplicationInstance.Context;
            var service = new ArticlesService(page);
            return service.GetArticles();
        }).ContinueWith(t => SetResult(t, "articles"));

So we are copying the context object onto the new thread that is spawned from Task factory. This context.Items is used again in the thread wherever necessary. Say for ex:

public class SomeClass
  {
    internal static int StreamID
    {
        get
        {
            if (HttpContext.Current != null)
            {
                return (int)HttpContext.Current.Items["StreamID"];
            }
            else
            {
                return DEFAULT_STREAM_ID;
            }
        }
    }

This runs fine as long as number of parallel requests are optimal. My questions are as follows:

1. When the load is more and there are too many parallel requests, I notice that HttpContext.Current.Items is empty. I am not able to figure out a reason for this and this causes all the null reference exceptions.

2. How do we make sure it is not null ? Any workaround if present ?

NOTE: I read through in StackOverflow and people have questions like HttpContext.Current is null - but in my case it is not null and its empty. I was reading one more article where the author says that sometimes request object is terminated and it may cause problems since dispose is already called on objects. I am doing a copy of Context object - its just a shallow copy and not a deep copy.

Answer 1

Your problem is that a instance members of the HttpContext are not thread safe:

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

When accessing it the way you are doing (multiple threads) you need to do your own synchronization.

static object locker = new object();
get
{
    lock (locker)
    {
        if (HttpContext.Current != null)
        {
            return (int)HttpContext.Current.Items["StreamID"];
        }
        else
        {
            return DEFAULT_STREAM_ID;
        }
    }
}

MSDN: system.web.httpcontext