The cross-thread usage of “HttpContext.Current” property and related things

by

There are four things working together to cause the behavior you are asking about:

  1. HttpContext is an instance object whose reference can be found in HttpContext.Current
  2. Thread is also an instance object whose reference can be found in Thread.CurrentThread
  3. Thread.CurrentThread is static but references a different Thread object in every thread
  4. HttpContext.Current actually points to Thread.CurrentThread.ExecutionContext.IllogicalCallContext.HostContext

Conclusions we can draw from the above givens:

  1. Because HttpContext is an instance object and not static we need its reference to access it
  2. Because HttpContext.Current actually points to a property on Thread.CurrentThread, changing Thread.CurrentThread to a different object will likely change HttpContext.Current
  3. Because Thread.CurrentThread‘ changes when switching threads, HttpContext.Current also changes when switching threads (in this case HttpContext.Current becomes null).

Bringing this all together, what causes HttpContext.Current to not work in a new Thread? The Thread.CurrentThread reference change, which happens when switching threads, changes the HttpContext.Current reference, which prevents us from getting to the HttpContext instance we want.

To reiterate, the only magic thing going on here is Thread.CurrentThread referencing a different object in every Thread. HttpContext works just like any other instance object. Since threads in the same AppDomain can reference the same objects, all we have to do is pass a reference for HttpContext to our new thread. There is no context info to load or anything like that. (there are some fairly serious potential gotchas with passing around HttpContext to other threads but nothing to prevent you from doing it).

A few final side notes I came across while researching:

  1. In some cases a Thread’s ExecutionContext is ‘flowed’ (copied) from one Thread to another. Why then is HttpContext not ‘flowed’ to our new Thread? Because HttpContext doesn’t implement the ILogicalThreadAffinative interface. A class stored in the ExecutionContext is only flowed if it implements ILogicalThreadAffinative.

  2. How does ASP.NET move HttpContext from Thread to Thread (Thread-Agility) if it isn’t flowed? I’m not entirely sure, but it looks like it might pass it in HttpApplication.OnThreadEnter().

More Related Contents:

  1. Can I use threads to carry out long-running jobs on IIS?
  2. BackgroundWorker thread in ASP.NET
  3. How is ASP.NET multithreaded?
  4. Using ThreadPool.QueueUserWorkItem in ASP.NET in a high traffic scenario
  5. Can the ViewBag name be the same as the Model property name in a DropDownList?
  6. IIS AppPoolIdentity and file system write access permissions
  7. Losing Session State
  8. Close/kill the session when the browser or tab is closed
  9. Asp.Net UpdatePanel in Gridview Jquery DatePicker
  10. ASP.NET dynamically created controls and Postback
  11. Best way to stream files in ASP.NET
  12. Application vs Session vs Cache
  13. ListItems attributes in a DropDownList are lost on postback?
  14. Sys is undefined
  15. @Html.DisplayFor – DateFormat (“mm/dd/yyyy”)
  16. ASP.NET Application state vs a Static object
  17. Form Authentication – Cookie replay attack – protection
  18. Configure Microsoft.AspNet.Identity to allow email address as username
  19. double escape sequence inside a url : The request filtering module is configured to deny a request that contains a double escape sequence
  20. Data Annotations for validation, at least one required field?
  21. Could not load file or assembly or one of its dependencies. Access is denied. The issue is random, but after it happens once, it continues
  22. Search Engine – Lucene or Solr
  23. Is there a way to keep a page from rendering once a person has logged out but hit the “back” button?
  24. Quartz.NET scheduler doesn’t fire jobs/triggers once deployed
  25. Visual Studio Do not open new browser instance
  26. Duplicate ‘Content’ items were included. The .NET SDK includes ‘Content’ items from your project directory by default
  27. Does the Razor View Engine work for Mono?
  28. IIS and Static content?
  29. Differences between App.Config and Web.Config?
  30. How to do AsyncPostBackTrigger for the LinkButton in the Repeater

Leave a Comment