How can I reference HttpContext from async methods in ASP.NET Web API?

Question

I'm working on a Web API, which purely has synchronous code running. I wanted to turn certain methods async and run some parallel tasks. The problem is that after turning the methods async, the HttpContext.Current becomes null, when it's referred to subsequently in the async methods.

The Web API has a filter put on certain controller methods that looks something like this:

public class AuthenticationTokenFilter : ActionFilterAttribute
{
    public override void OnActionExecuting(HttpActionContext actionContext)
    {
        var email = // Get some stuff from the request and do some things with it.
        var id = // Get some stuff from the request and do some things with it.

        System.Web.HttpContext.Current.Items.Add("Property1", email);
        System.Web.HttpContext.Current.Items.Add("Property2", id);
    }

Subsequently, an injected class called something like a ContextProvider will refer directly to the HttpContext to retrieve these properties. However, when calling the provider from the async methods, the context is null, and we get a null reference is exception.

I've read several posts on this, and the solution is not obvious to me. I've tried setting the following in the configuration file:

<add key="aspnet:UseTaskFriendlySynchronizationContext" value="true" />

and I've ensured that a higher version than .NET 4.5 is targeted(it uses 4.6.1).

Others seem to suggest that you can invoke the async methods with the HttpContext, but I don't know how to do it, and would prefer not to have to invoke the HttpContext in every single method awaited in this chain.

score 0 · Answer 1 · answered Oct 10 '19 at 14:59

You didn't show your code where you actually use the async methods, but my guess is that you're using .ConfigureAwait(false). Some people sprinkle .ConfigureAwait(false) everywhere, but it's not that simple.

The parameter to ConfigureAwait is:

continueOnCapturedContext Boolean

true to attempt to marshal the continuation back to the original context captured; otherwise, false.

HttpContext is only available on the same context where the request started. So you must return to the same context if you intend on using HttpContext after awaiting an asynchronous method.

So if you do have .ConfigureAwait(false) in your code, remove it.

There is a more thorough explanation in this answer.

How can I reference HttpContext from async methods in ASP.NET Web API?

1 Answers1