HttpContext.Current is null in an asynchronous Callback

Question

Trying to access the HttpContext.Current in a method call back so can I modify a Session variable, however I receive the exception that HttpC

Answer 1

Here's a class-based solution that is working for simple cases so far in MVC5 (MVC6 supports a DI-based context).

using System.Threading;
using System.Web;

namespace SomeNamespace.Server.ServerCommon.Utility
{
    /// <summary>
    /// Preserve HttpContext.Current across async/await calls.  
    /// Usage: Set it at beginning of request and clear at end of request.
    /// </summary>
    static public class HttpContextProvider
    {
        /// <summary>
        /// Property to help ensure a non-null HttpContext.Current.
        /// Accessing the property will also set the original HttpContext.Current if it was null.
        /// </summary>
        static public HttpContext Current => HttpContext.Current ?? (HttpContext.Current = __httpContextAsyncLocal?.Value);

        /// <summary>
        /// MVC5 does not preserve HttpContext across async/await calls.  This can be used as a fallback when it is null.
        /// It is initialzed/cleared within BeginRequest()/EndRequest()
        /// MVC6 may have resolved this issue since constructor DI can pass in an HttpContextAccessor.
        /// </summary>
        static private AsyncLocal<HttpContext> __httpContextAsyncLocal = new AsyncLocal<HttpContext>();

        /// <summary>
        /// Make the current HttpContext.Current available across async/await boundaries.
        /// </summary>
        static public void OnBeginRequest()
        {
            __httpContextAsyncLocal.Value = HttpContext.Current;
        }

        /// <summary>
        /// Stops referencing the current httpcontext
        /// </summary>
        static public void OnEndRequest()
        {
            __httpContextAsyncLocal.Value = null;
        }
    }
}

To use it can hook in from Global.asax.cs:

    public MvcApplication() // constructor
    {            
        PreRequestHandlerExecute += new EventHandler(OnPreRequestHandlerExecute);
        EndRequest += new EventHandler(OnEndRequest);
    } 

    protected void OnPreRequestHandlerExecute(object sender, EventArgs e)
    {
        HttpContextProvider.OnBeginRequest();   // preserves HttpContext.Current for use across async/await boundaries.            
    }

    protected void OnEndRequest(object sender, EventArgs e)
    {
        HttpContextProvider.OnEndRequest();
    }

Then can use this in place of HttpContext.Current:

    HttpContextProvider.Current

There may be issues as I currently do not understand this related answer. Please comment.

Reference: AsyncLocal (requires .NET 4.6)

Answer 2

Please see the following article for an explanation on why the Session variable is null, and possible work arounds

http://adventuresdotnet.blogspot.com/2010/10/httpcontextcurrent-and-threads-with.html

quoted from the from the article;

the current HttpContext is actually in thread-local storage, which explains why child threads don’t have access to it

And as a proposed work around the author says

pass a reference to it in your child thread. Include a reference to HttpContext in the “state” object of your callback method, and then you can store it to HttpContext.Current on that thread

Answer 3

When using threads or an async function, HttpContext.Current is not available.

Try using:

HttpContext current;
if(HttpContext != null && HttpContext.Current != null)
{
  current = HttpContext.Current;
}
else
{
    current = this.CurrentContext; 
    //**OR** current = threadInstance.CurrentContext; 
}

Once you set current with a proper instance, the rest of your code is independent, whether called from a thread or directly from a WebRequest.