Is there any danger in using ConfigureAwait(false) in WebApi or MVC controllers?

Question

Say I have two scenarios:

1) WebApi Controller

    [System.Web.Http.HttpPost]     [System.Web.Http.AllowAnonymous]     [Route("api/registerMobile")]     public async Task<HttpResponseMessage> RegisterMobile(RegisterModel model)     {         var registerResponse = await AuthUtilities.RegisterUserAsync(model, _userService, User);         if (registerResponse.Success) {             var response = await _userService.GetAuthViewModelAsync(model.Username, User);             return Request.CreateResponse(HttpStatusCode.OK, new ApiResponseDto() { Success = true, Data = response });         }         else {             return Request.CreateResponse(HttpStatusCode.OK, registerResponse);         }      } 

2) MVC Controller

    [Route("public")]     public async Task<ActionResult> Public()     {         if (User.Identity.IsAuthenticated)         {             var model = await _userService.GetAuthViewModelAsync(User.Identity.Name);             return View("~/Views/Home/Index.cshtml", model);         }         else         {             var model = await _userService.GetAuthViewModelAsync(null);             return View("~/Views/Home/Index.cshtml", model);         }     } 

I've been reading up on when I should use ConfigureAwait and it seems like I should use ConfigureAwait(false) on ALL of my async calls that are not tied directly to the UI. I don't know what that means though... should I be using .ConfigureAwait(false) on all of the above await calls?

I'm looking for some unambiguous guidelines around when exactly I should be using it.

This question is NOT the same as the Best practice to call ConfigureAwait for all server-side code - I am looking for a straightforward answer on the use-case for this method in the context of WebApi and MVC, not as general C#.

Answer 1

it seems like I should use ConfigureAwait(false) on ALL of my async calls that are not tied directly to the UI.

Not quite. That guideline doesn't make sense here, since there is no UI thread.

The parameter passed to ConfigureAwait is continueOnCapturedContext, which explains more clearly the scenario. You want to use ConfigureAwait(false) whenever the rest of that async method does not depend on the current context.

In ASP.NET 4.x, the "context" is the request context, which includes things like HttpContext.Current and culture. Also - and this is the undocumented part - a lot of the ASP.NET helper methods do depend on the request context.

(Side note: ASP.NET Core no longer has a "context")

should I be using .ConfigureAwait(false) on all of the above await calls?

I haven't heard any firm guidance on this, but I suspect it's OK.

In my own code, I never use ConfigureAwait(false) in my controller action methods, so that they complete already within the request context. It just seems more right to me.