Set Thread.CurrentPrincipal Asynchronously?

Question

Using ASP.NET WebAPI, during authentication, Thread.CurrentPrincipal is set so that controllers can later use the ApiController.User property.

If that authentication step becomes asynchronous (to consult another system), any mutation of CurrentPrincipal is lost (when the caller's await restores the synchronization context).

Here's a very simplified example (in the real code, authentication happens in an action filter):

using System.Diagnostics;
using System.Security.Principal;
using System.Threading;
using System.Threading.Tasks;

public class ExampleAsyncController : System.Web.Http.ApiController
{
    public async Task GetAsync()
    {
        await AuthenticateAsync();

        // The await above saved/restored the current synchronization
        // context, thus undoing the assignment in AuthenticateAsync(). 
        Debug.Assert(User is GenericPrincipal);
    }

    private static async Task AuthenticateAsync()
    {
        // Save the current HttpContext because it's null after await.
        var currentHttpContext = System.Web.HttpContext.Current;

        // Asynchronously determine identity.
        await Task.Delay(1000);
        var identity = new GenericIdentity("<name>");

        var roles = new string[] { };
        Thread.CurrentPrincipal = new GenericPrincipal(identity, roles);
        currentHttpContext.User = Thread.CurrentPrincipal;
    }
}

How do you set Thread.CurrentPrincipal in an async function such that the caller's await doesn't discard that mutation when restoring the synchronization context?

Answer 1

You have to set HttpContext.Current.User as well. See this answer and this blog post for more info.

Update: Also ensure you are running on .NET 4.5 and have UserTaskFriendlySynchronizationContext set to true.