added middleware with error handling example

Author: Marcin Liszewski

README.md

@@ -1,4 +1,4 @@
-# 319 Samples for ASP.NET Core 3.1.300 and ASP.NET Core 5 RC1
+# 320 Samples for ASP.NET Core 3.1.300 and ASP.NET Core 5 RC1
 
 ## People of Beirut needs our help. [Please support them](https://lebanoncrisis.carrd.co/#donate).
 
@@ -21,7 +21,7 @@ Hi Nuget visitors, if you have problem finding the sample you are looking for, p
 
 | Section | | | |
 |-----------------------------------------------------------------|------|-----------------------------------------------------------------------------------------------------------------|------|
-| [Blazor Client Side (Web Assembly)](/projects/blazor/README.md) (Components, Data Binding) | (20) | [Middleware](/projects/middleware) | (13) |
+| [Blazor Client Side (Web Assembly)](/projects/blazor/README.md) (Components, Data Binding) | (20) | [Middleware](/projects/middleware) | (14) |
 | [Blazor Server Side](/projects/blazor-ss) (Localization) | (15) | [MVC](/projects/mvc) (Localization, Routing, Razor Class Library, Tag Helpers, View Component, etc) | (46) |
 | [Caching](/projects/caching) | (5) | [Razor Pages](/projects/razor-pages) | (9) |
 | [Configurations](/projects/configurations) | (7) | [Orchard Core](/projects/orchard-core) | (4) |

projects/middleware/README.md

@@ -59,3 +59,7 @@
  * [Middleware 12](/projects/middleware/middleware-12)
 
  Contrast the usage of `MapWhen` (which branch execution) and `UseWhen` (which doesn't branch execution) in configuring your Middleware.
+
+ * [Middleware 13](/projects/middleware/middleware-13)
+
+ Demonstrate how to implement basic error handling mechanism and how to return error object (same for all api exceptions).

projects/middleware/middleware-13/middleware-13.csproj

@@ -0,0 +1,9 @@
+<Project Sdk="Microsoft.NET.Sdk.Web">
+
+ <PropertyGroup>
+ <TargetFramework>netcoreapp3.1</TargetFramework>
+ <RootNamespace>middleware_13</RootNamespace>
+ </PropertyGroup>
+
+
+</Project>

projects/middleware/middleware-13/src/Program.cs

@@ -0,0 +1,99 @@
+using System;
+using System.Net;
+using System.Text.Json;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Builder;
+using Microsoft.AspNetCore.Hosting;
+using Microsoft.AspNetCore.Http;
+using Microsoft.Extensions.Hosting;
+using Microsoft.Extensions.Logging;
+
+namespace middleware_13
+{
+ public class Program
+ {
+ public static void Main(string[] args)
+ {
+ CreateHostBuilder(args).Build().Run();
+ }
+
+ public static IHostBuilder CreateHostBuilder(string[] args) =>
+ Host.CreateDefaultBuilder(args)
+ .ConfigureWebHostDefaults(webBuilder =>
+ {
+ webBuilder.UseStartup<Startup>();
+ });
+ }
+ 
+ public class Startup
+ {
+ public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
+ {
+ app.UseMiddleware<ErrorHandlingMiddleware>();
+ }
+ }
+
+ public interface ICustomApiException
+ {
+ HttpStatusCode HttpStatus { get; }
+ string HttpMessage { get; }
+ }
+ 
+ public class ExampleApiException : Exception, ICustomApiException
+ {
+ public HttpStatusCode HttpStatus => HttpStatusCode.Forbidden;
+ public string HttpMessage => "Resource cannot be accessed.";
+
+ public ExampleApiException(string message) : base(message)
+ {
+ }
+ }
+
+ public class ErrorDto
+ {
+ public int Status { get; set; }
+ public string Message { get; set; }
+ }
+
+ public class ErrorHandlingMiddleware : IMiddleware
+ {
+ private readonly ILogger<ErrorHandlingMiddleware> _logger;
+
+ public ErrorHandlingMiddleware(ILogger<ErrorHandlingMiddleware> logger)
+ {
+ _logger = logger;
+ }
+
+ public async Task InvokeAsync(HttpContext context, RequestDelegate next)
+ {
+ try
+ {
+ await next.Invoke(context);
+ }
+ catch (Exception ex)
+ {
+ _logger.LogError(ex, ex.Message);
+
+ var httpStatus = HttpStatusCode.InternalServerError;
+ var httpMessage = "Internal Server Error";
+ 
+ if (ex is ICustomApiException customException)
+ {
+ httpStatus = customException.HttpStatus;
+ httpMessage = customException.HttpMessage;
+ }
+ 
+ context.Response.StatusCode = (int)httpStatus;
+ context.Response.ContentType = "application/json";
+
+ var error = new ErrorDto()
+ {
+ Status = (int)httpStatus,
+ Message = httpMessage
+ };
+
+ await context.Response.WriteAsync(JsonSerializer.Serialize(error));
+ }
+ }
+ }
+}