WebAPI how do I deserialize exceptions on the (.NET) client side

Question

I have a webAPI 2 controller, Currently it is throwing an exception, and this is being sent back to the client in a response message. 500 internal server error, the exception it's self is serialised and I can see the json representation of it in fiddler. (json pasted below)

I have been trying to find a way to de-serialize this exception back to a an .NET Exception object, but keep getting the error:

"Member 'ClassName' was not found."

currently in my client I am trying to deserialise the exception via the following code.

if (apiResponse.ResponseCode.Equals(500)) // Unhandled exception on server
{
    var exceptionObject = await response.Content.ReadAsAsync<Exception>();
}

How do I properly de-serialize this exception object on my client side application? ideally I should be able to get back the original System.InvalidOperationException` object.

Below is the Response I am trying to de-serialize as captured in fiddler:

HTTP/1.1 500 Internal Server Error Cache-Control: no-cache Pragma: no-cache Content-Type: application/json; charset=utf-8 Expires: -1 Server: Microsoft-IIS/10.0 X-AspNet-Version: 4.0.30319 X-Powered-By: ASP.NET Date: Wed, 30 Sep 2015 13:43:42 GMT Content-Length: 1556

{"Message":"An error has occurred.","ExceptionMessage":"UrlHelper.Link must not return null.","ExceptionType":"System.InvalidOperationException","StackTrace":" at System.Web.Http.Results.CreatedAtRouteNegotiatedContentResult1.Execute()\r\n at System.Web.Http.Results.CreatedAtRouteNegotiatedContentResult1.ExecuteAsync(CancellationToken cancellationToken)\r\n at System.Web.Http.Controllers.ApiControllerActionInvoker.d__0.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)\r\n at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n at System.Web.Http.Controllers.ActionFilterResult.d__2.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)\r\n at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n at System.Web.Http.Filters.AuthorizationFilterAttribute.d__2.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)\r\n at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n at System.Web.Http.Dispatcher.HttpControllerDispatcher.d__1.MoveNext()"}

Why would you want that? This way you'll be tightly coupling your implementation to server. — Sergei Rogovtcev
– Sergei Rogovtcev, Commented Sep 30, 2015 at 14:09

victorvartan · Accepted Answer · 2017-07-29 10:56:10Z

6

Thanks to the comment from Serg Rogovtsev, I now know that what you have is not an Exception but a HttpError. Therefore, you'll have to handle it as such:

var exceptionObject = await response.Content.ReadAsAsync<System.Web.Http.HttpError>();

I'm still not convinced that's a good idea, though. A 500 status code generally means that something went unexpectedly wrong and there was nothing there to catch it on the server side. If you have control over the server, I'd strongly suggest to handle such errors there. The client is unable to do anything about it anyway, so all it needs is some kind of error notification.

Old answer (before the comment about HttpError):

A serialized Exception looks something like this:

{
    "ClassName":"System.Exception",
    "Message":"some exception",
    "Data":null,
    "InnerException":null,
    "HelpURL":null,
    "StackTraceString":null,
    "RemoteStackTraceString":null,
    "RemoteStackIndex":0,
    "ExceptionMethod":null,
    "HResult":-2146233088,
    "Source":null,
    "WatsonBuckets":null
}

What you have is just a formatted error message that contains information about the exception.

By the way: I wouldn't try to catch server-side exceptions on the client. You need to handle errors on the server and provide the client with enough information to inform the user about what happened.

edited Jul 29, 2017 at 10:56

victorvartan

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answered Sep 30, 2015 at 14:28

germi

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3 Comments

Sergei Rogovtcev Over a year ago

Yep, WebAPI wraps exceptions in HttpError.

germi Over a year ago

@SergRogovtsev ah, good to know. So John could deserialize that into an HttpError.

John Over a year ago

Yeah, after thinking about this, I decided also that it was a bad idea. I now just Read the content as a string and wrap in a client sided exception object.

John · Accepted Answer · 2016-03-17 14:39:38Z

I found a good way to get the json exception I want. I got this while looking through the samples on the ASPNET git repository.

In the ASPNET Startup.cs Configure method add the following (for dev environment only since as noted in other answers a 500 is probably not useful to the client)

public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
{
    if (env.IsDevelopment())
    {
        app.Use(async (context, next) =>
        {
             try
             {
                  await next();
             }
             catch (Exception ex)
             {
                  if (context.Response.HasStarted)
                  {
                      throw;
                  }
                  context.Response.StatusCode = 500;
                  await context.Response.WriteJsonExcpetionAsync(ex);
              }
        });
    }

    // Other config logic...
}

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