错误处理和try / catch块与异步方法

Question

I'm writing an ASP.Net Core 2 MVC application in Visual Studio 2017 Community Edition. I'm having trouble understanding how try-catch blocks behave when dealing with asynchronous methods. Below are two snippets, an action in a controller and the method being called by the action.

Let me start by saying that the program works as intended. The problem is that when I deliberately create an error I would expect the middleware to display the predetermined Error page. This also works as intended in a synchronous version of the same code.

With this asynchronous version instead, the browser does not return any error, not even an HTTP500 Internal Server error page, it just keeps waiting for something to happen. If I then look at the WebServer Output, I see the that the exception was thrown and I even read that the error page view is run, when in fact it wasn't.

Would someone be kind enough to explain what I'm missing here? Thank you!

This is the action. I skipped a lot of content in the method because it should be irrelevant and it is really long. Also, I don't think it is relevant, but then what do I know... so pls note that this action returns a partial view. The action itself is called by an ajax request in another action.

     // Variance Report -- POST
     [HttpPost]
     [Authorize]
     [ValidateAntiForgeryToken]
     public async Task<IActionResult> Variance(VarianceModel model)
     {
         try
         {
             // Get the report data
             var varianceReport = await _report.GetVarianceReportAsync(model);

             // String that will display the current selection
             ViewBag.Parameters = model.parameterDescription;

             // Return the partial view
             return PartialView("_VariancePartial", varianceReport);
         }
         catch (Exception)
         {
             throw;
         }
     }

Here is the method being called:

     // REPORT
     public async Task<VarianceModel> GetVarianceReportAsync(VarianceModel model)
     {
         try
         { 
             // Do a lot of stuff here... then I deliberately try to
             // divide by zero so as to get an error:
                         try
                         {
                             VariancePercent = (VarianceValue / 0)
                         }
                         catch (Exception)
                         {
                             throw;
                         }
             // Do more stuff here as well...
                 }
             return model;
         }
         catch (Exception)
         {
             throw;
         }
     }

and finally, this is the output from the webserver:

[...bla bla bla...]
WebApp> fail: Microsoft.AspNetCore.Diagnostics.ExceptionHandlerMiddleware[0]
WebApp>       An unhandled exception has occurred: Attempted to divide by zero.
WebApp> System.DivideByZeroException: Attempted to divide by zero.
[...bla bla bla...]
WebApp> info: Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker[1]
WebApp>       Executing action method WebApp.Controllers.HomeController.Error ( WebApp) with arguments ((null)) - ModelState is Valid
WebApp> info: 
Microsoft.AspNetCore.Mvc.ViewFeatures.Internal.ViewResultExecutor[1]
WebApp>       Executing ViewResult, running view at path /Views/Shared/Error.cshtml.
WebApp> info: Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker[2]
WebApp>       Executed action WebApp.Controllers.HomeController.Error (WebApp) in 1.8921ms
WebApp> info: Microsoft.AspNetCore.Hosting.Internal.WebHost[2]
WebApp>       Request finished in 66679.065ms 500 text/html; charset=utf-8

Answer 1

You're tossing an exception but the page doesn't have a way to surface it. Rather than just throwing the exception, you should return an actual HTTP response:

[HttpPost]
[Authorize]
[ValidateAntiForgeryToken]
public async Task<IActionResult> Variance(VarianceModel model)
{
    try
    {
        // Get the report data
        var varianceReport = await _report.GetVarianceReportAsync(model);

        // String that will display the current selection
        ViewBag.Parameters = model.parameterDescription;

        // Return the partial view
        return PartialView("_VariancePartial", varianceReport);
    }
    catch (Exception ex)
    {
        return new ObjectResult(ex);
    }
}

// REPORT
public async Task<VarianceModel> GetVarianceReportAsync(VarianceModel model)
{
    try
    {
        // divide by zero so as to get an error:
        VariancePercent = (VarianceValue / 0);
        return model;
    }
    catch (Exception)
    {
        throw;
    }
}

This won't be a very clean response however. Ideally, you should create a partial view for the error state and return that:

public async Task<IActionResult> Variance(VarianceModel model)
{
    try
    {
        // Get the report data
        var varianceReport = await _report.GetVarianceReportAsync(model);

        // String that will display the current selection
        ViewBag.Parameters = model.parameterDescription;

        // Return the partial view
        return PartialView("_VariancePartial", varianceReport);
    }
    catch (Exception ex)
    {
        return PartialView("_ErrorPartial", ex);
    }
}