Parallel Tasks and the HttpContext

9 Comments

1. Me My Self says:

   14 February, 2012 at 15:02

   So, what do you suggest as caching method inside an asp.net application ?

   Reply
   1. Colin Mackay says:

      14 February, 2012 at 19:31

      I’m not sure what this post has to do with caching… But there are many mechanisms built in to ASP.NET for caching and in to the .NET Framework in general. It really depends on what you want to achieve.

      Reply
2. EZ coding says:

   15 February, 2012 at 05:07

   Actually I am using HttpContext.Current to read Session /Cache /Request collections from classes other than code behind. Never had any concurrency problems.
   Hit the first problem today while implementing parallel task that call function that internally access HttpContext.Current. Apparently it is not shared between ASP and parallel thread

   Reply
   1. Colin Mackay says:

      15 February, 2012 at 14:07

      HttpContext.Current is set at the thread level. When accessing the HttpContext.Current from classes other than the “code behind” you are still on the same thread that the request is using. As soon as you create new threads then HttpContext.Current returns the context that is current to the thread you are on. Since the parallel task is not the same thread there is no current HttpContext.

      It isn’t so much that you “lose” the HttpContext.Current on a parallel task, it is more that you never had one in the first place.

      Reply
      1. neha says:

         15 November, 2012 at 19:54

         How can I fix this? I am using Task.Factory.StartNew to calling methods in classes and populate my viewmodel. In one of the methods, I need context information as well, which is getting set to null as there is no HttpContext to that thread. How do I pass httpcontext?
         Appreciate your help

         Reply
         1. Colin Angus Mackay says:

            15 November, 2012 at 21:35

            As I said in the article, I wouldn’t pass the HttpContext into parallel tasks because I don’t know how thread safe it is. Having said that, if all you are going to do is read only from the HTTP Context then there is no real risk.

            If you are using Task.Factory.StartNew() to initiate the parallel task then you can pass it in to the task just like any other parameter. Something like this, perhaps:

            // Allow the current context to be used as a closure.
            HttpContext current = HttpContext.Current;
            
            // Start the task
            Task.Factory.StartNew( () => DoStuff(current));
            
            // Perform the work of the task
            public void DoStuff(HttpContext context)
            {
              // Do stuff with the context.
            }
            

            I’ve not tested this. But I imagine this would work. The reason for depositing the HttpContext.Current in a variable before starting the task is to ensure that it can be used as a closure in the lambda expression. If we didn’t do that then trying to call HttpContext.Current in the lambda expression would always return null because by that point you are in the thread created by the parallel task.

            However, as I’ve said in this post ASP.NET (MVC or WebForms) is already heavily multi-threaded so you need to ask your self is your use case sufficiently justified to launch more threads?

            Reply
            1. neha says:

               19 November, 2012 at 14:34

               Thanks alot for your answer. I understood this completely now.
               Great post!! 🙂

3. kln says:

   24 February, 2014 at 03:41

   Thanks for sharing!!

   Reply
4. IWC Cousteau says:

   13 November, 2014 at 06:28

   Thanks. I hope that’s the case!

   Reply