.Net Core: Return IActionResult from a custom Exception Middleware

Question

I have created a new Exception middleware in my .Net Core application. All the exceptions throughout the application are captured and logged here. What I want is to return a IActionResult type like InternalServerError() or NotFound() from the Exception Middleware and not do response.WriteAsync as below.

Controller Method:

   public async Task<IActionResult> Post()
    {
        //Do Something
        return Ok();
    }

Middleware:

public class ExceptionMiddleware
    {
        private readonly RequestDelegate _next;
        public ExceptionMiddleware(RequestDelegate next)
        {
            _next = next;
        }

        public async Task Invoke(HttpContext context)
        {
            try
            {
                await _next.Invoke(context);
            }
            catch (Exception ex)
            {
                await HandleExceptionAsync(context, ex);
            }
        }

        private async Task HandleExceptionAsync(HttpContext context, Exception exception)
        {
            var response = context.Response;
            var statusCode = (int)HttpStatusCode.InternalServerError;
            var message = exception.Message;
            var description = exception.Message;

            response.ContentType = "application/json";
            response.StatusCode = statusCode;

            await response.WriteAsync(JsonConvert.SerializeObject(new ErrorResponse
            {
                Message = message,
                Description = description
            }));
        }
    }

Answer 1

IActionResult is a thing from MVC, so it is only available within the MVC pipeline (including Razor Pages). Just before the MVC middleware terminates, it will execute those action results using ExecuteResultAsync. That method is then responsible of writing that response to HttpContext.Response.

So in custom middleware, you cannot just set an action result and have it executed, since you are not running within the MVC pipeline. However, with that knowledge, you can simply execute the result yourself.

Let’s say you want to execute a NotFoundResult which is what Controller.NotFound() creates. So you create that result and call ExecuteResultAsync with an . That executor will be able to execute that result object and write to the response:

var result = new NotFoundResult();
await result.ExecuteResultAsync(new ActionContext
{
    HttpContext = context
});

Answer 2

That's not really possible due to where IActionResult and middleware sit in relation to one another in the architecture. Middleware sits much lower, and so it can't reach further up the stack to IActionResult. Here's an answer that talks more about it: https://stackoverflow.com/a/43111292/12431728

Answer 3

What you're trying to do can be done by simply adding this line:

app.UseStatusCodePagesWithReExecute("/Public/Error", "?statusCode={0}");

to the Configure method in the Startup.cs. Then you can create your Public Controller with Error method that does the following:

[AllowAnonymous]
public IActionResult Error(int? statusCode = null)
{
    // Retrieve error information in case of internal errors.
    var error = HttpContext.Features.Get<IExceptionHandlerFeature>()?.Error;
    var path = HttpContext.Features.Get<IExceptionHandlerPathFeature>()?.Path;

    // TODO: Redirect here based on your status code or perhaps just render different views for different status codes.
}

There is also another middleware that allows you to do a similar thing:

app.UseStatusCodePages(async context =>
{
    if (context.HttpContext.Response.StatusCode == 401)
    {
        context.HttpContext.Response.Redirect("Errors/Unauthorized/");
    }
    else if (context.HttpContext.Response.StatusCode == 500)
    {
        // TODO: Redirect for 500 and so on...
    }
 });