MediatrErrorHandlingMiddleware.cs

using Microsoft.AspNetCore.Http;
using Newtonsoft.Json;
using System;
using System.Linq;
using System.Net;
using System.Threading.Tasks;

namespace Infrastructure.Middleware
{
	public class MediatrErrorHandlingMiddleware
	{
		private readonly RequestDelegate next;

		public MediatrErrorHandlingMiddleware(RequestDelegate next)
		{
			this.next = next;
		}

		public async Task Invoke(HttpContext context /* other scoped dependencies */)
		{
			try
			{
				// must be awaited
				await next(context);
			}
			catch (Exception ex)
			{
                if (ShouldHandleException(ex))
                {
                    await HandleExceptionAsync(context, ex);
                }
                else
                {
                    throw;
                }
			}
		}

        private static bool ShouldHandleException(Exception exception)
        {
            if (exception is MediatrUnauthorizedAccessException) return true;
            if (exception is MediatrValidationException) return true;
            if (exception is AggregateException)
            {
                return (exception as AggregateException).Flatten().InnerExceptions
                    .Any(x => x is MediatrUnauthorizedAccessException || x is MediatrValidationException);
            }

            return false;
        }

		private static Task HandleExceptionAsync(HttpContext context, Exception exception)
		{
			Exception e;

			if (exception is AggregateException)
			{
				e = (exception as AggregateException).Flatten().InnerExceptions.FirstOrDefault();
			}
			else
			{
				e = exception;
			}

			if (e is MediatrUnauthorizedAccessException)
			{
				var error = new
				{
					message = e.Message,
					exception = e.GetType().Name
				};
				return WriteExceptionAsync(context, HttpStatusCode.Unauthorized, error);
			}
			else if (e is MediatrValidationException)
			{
				MediatrValidationException mve = e as MediatrValidationException;
				string message = "<span>Validation failed</span>" + "<ul>" + string.Join("", mve.Errors.Select(x => "<li>" + x.ErrorMessage.ToString() + "</li>")) + "</ul>";
				var fields = mve.Errors.Select(x => x.PropertyName);

				var error = new
				{
					message = message,
					fields = fields,
					exception = e.GetType().Name
				};
				return WriteExceptionAsync(context, HttpStatusCode.BadRequest, error);
			}

            return Task.FromResult(0);
        }

        private static Task WriteExceptionAsync(HttpContext context, HttpStatusCode code, object error)
		{
			var response = context.Response;
			response.ContentType = "application/json";
			response.StatusCode = (int)code;
			return response.WriteAsync(JsonConvert.SerializeObject(error));
		}
	}
}