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
{
    /// 

    /// Preserve HttpContext.Current across async/await calls.  
    /// Usage: Set it at beginning of request and clear at end of request.
    /// 
    static public class HttpContextProvider
    {
        /// 

        /// Property to help ensure a non-null HttpContext.Current.
        /// Accessing the property will also set the original HttpContext.Current if it was null.
        /// 
        static public HttpContext Current => HttpContext.Current ?? (HttpContext.Current = __httpContextAsyncLocal?.Value);

        /// 

        /// 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.
        /// 
        static private AsyncLocal __httpContextAsyncLocal = new AsyncLocal();

        /// 

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

        /// 

        /// Stops referencing the current httpcontext
        /// 
        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)