How can I return a response in ASP.NET Core MVC middleware using MVC's content negotiation? - .net-core

Question

I have some ASP.NET Core MVC middleware to catch unhandled exceptions that I would like to return a response from.
While it is easy to just httpContext.Response.WriteAsync to write a string and e.g. use JsonSerializer to serialise an object to a string, I would like to use the standard serialisation settings and content negotiation so that if I change my default output formatting to XML or a text/xml accept header is sent when I have multiple output formatters configured then XML is returned, as it does if I return an ObjectResult from a controller.
Does anyone know how this can be achieved in middleware?
Here is my code so far which only writes JSON:
public class UnhandledExceptionMiddleware
{
private readonly RequestDelegate _next;
private readonly IOutputFormatter _outputFormatter;
private readonly IHttpResponseStreamWriterFactory _streamWriterFactory;
public UnhandledExceptionMiddleware(RequestDelegate next, JsonOutputFormatter outputFormatter, IHttpResponseStreamWriterFactory streamWriterFactory)
{
_next = next;
_outputFormatter = outputFormatter;
_streamWriterFactory = streamWriterFactory;
}
public async Task Invoke(HttpContext context)
{
try
{
await _next(context);
}
catch (Exception ex)
{
await HandleExceptionAsync(context, ex);
}
}
private async Task HandleExceptionAsync(HttpContext context, Exception exception)
{
var error = new ErrorResultModel("Internal Server Error", exception.Message, exception.StackTrace);
context.Response.StatusCode = (int)HttpStatusCode.InternalServerError;
await _outputFormatter.WriteAsync(new OutputFormatterWriteContext(context, _streamWriterFactory.CreateWriter, typeof(ErrorResultModel), error));
}
}
where ErrorResultModel is defined as:
public class ErrorResultModel
{
public string ResultMessage { get; };
public string ExceptionMessage { get; };
public string ExceptionStackTrace { get; };
public ErrorResultModel(string resultMessage, string exceptionMessage, string exceptionStackTrace)
{
ResultMessage = resultMessage;
ExceptionMessage = exceptionMessage;
ExceptionStackTrace = exceptionStackTrace;
}
}

Answer 1

This is not possible in ASP.NET Core 2.0 MVC.
This will be possible in 2.1:
public static class HttpContextExtensions
{
private static readonly RouteData EmptyRouteData = new RouteData();
private static readonly ActionDescriptor EmptyActionDescriptor = new ActionDescriptor();
public static Task WriteResultAsync<TResult>(this HttpContext context, TResult result)
where TResult : IActionResult
{
if (context == null)
{
throw new ArgumentNullException(nameof(context));
}
var executor = context.RequestServices.GetService<IActionResultExecutor<TResult>>();
if (executor == null)
{
throw new InvalidOperationException($"No result executor for '{typeof(TResult).FullName}' has been registered.");
}
var routeData = context.GetRouteData() ?? EmptyRouteData;
var actionContext = new ActionContext(context, routeData, EmptyActionDescriptor);
return executor.ExecuteAsync(actionContext, result);
}
}
public class Program : StartupBase
{
public static Task Main(string[] args)
{
return BuildWebHost(args).RunAsync();
}
public static IWebHost BuildWebHost(string[] args)
{
return new WebHostBuilder().UseStartup<Program>().UseKestrel().Build();
}
public override void ConfigureServices(IServiceCollection services)
{
services.AddMvcCore().AddJsonFormatters();
}
public override void Configure(IApplicationBuilder app)
{
app.Use((ctx, next) =>
{
var model = new Person("Krisian", "Hellang");
var result = new ObjectResult(model);
return ctx.WriteResultAsync(result);
});
}
}
public class Person
{
public Person(string firstName, string lastName)
{
FirstName = firstName;
LastName = lastName;
}
public string FirstName { get; }
public string LastName { get; }
}