ASP.Net Core exception handling middleware

Question

I am trying to use middleware for exception handling in my ASP.Net Core 3.0 Web API project:

public class ErrorHandlingMiddleware
{
    private readonly RequestDelegate next;

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

    public async Task Invoke(HttpContext context)
    {
        try
        {
            await next(context);
        }
        catch (Exception ex)
        {
            await HandleException(context, ex);
        }
    }

    private static Task HandleException(HttpContext context, Exception ex)
    {
        HttpStatusCode code = HttpStatusCode.InternalServerError; // 500 if unexpected

        // Specify different custom exceptions here
        if (ex is CustomException) code = HttpStatusCode.BadRequest;

        string result = JsonConvert.SerializeObject(new { error = ex.Message });

        context.Response.ContentType = "application/json";
        context.Response.StatusCode = (int)code;

        return context.Response.WriteAsync(result);
    }
}

startup.cs

public class Startup
{
    public Startup(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public IConfiguration Configuration { get; }

    // This method gets called by the runtime. Use this method to add services to the container.
    public void ConfigureServices(IServiceCollection services)
    {
        // Add controllers with a route prefix
        services.AddControllers(x => { x.UseGeneralRoutePrefix($"api/v{Configuration["APIVersion"]}"); });
        services.AddSwaggerGen(c =>
        {
            c.SwaggerDoc("v0.1", new OpenApiInfo { Title = "My API", Version = "v0.1" });
        });
    }

    // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
    {
        if (env.IsDevelopment())
        {
            app.UseDeveloperExceptionPage();
        }

        app.UseHttpsRedirection();

        app.UseSwagger();
        app.UseSwaggerUI(c =>
        {
            c.SwaggerEndpoint("/swagger/v0.1/swagger.json", "My API V1");
        });

        app.UseMiddleware(typeof(ErrorHandlingMiddleware));

        app.UseRouting();

        app.UseAuthorization();

        app.UseEndpoints(endpoints =>
        {
            endpoints.MapControllers();
        });
    }
}

Controller

[HttpPost]
public ActionResult Method()
{
    if (condition)
        throw new CustomException();
    else
        return Ok();
}

but the exception thrown in the controller doesn't get handled by the middleware. What would be the correct way to use the middleware?

@KirkLarkin If the condition is not satisfied, the exception is thrown and the app just crashed. The HandleException() method from the middleware is never called. — Eutherpy
– Eutherpy, Commented Nov 12, 2019 at 15:16
Do you see the result of UseDeveloperExceptionPage kicking in? A nice HTML response showing the error, its stack-trace, etc? — Kirk Larkin
– Kirk Larkin, Commented Nov 12, 2019 at 15:18
Error handling middleware should be registered as early in the pipeline as possible. — Nkosi
– Nkosi, Commented Nov 12, 2019 at 15:19

Eutherpy · Accepted Answer · 2019-11-12 15:30:55Z

12

It seems that

app.UseDeveloperExceptionPage();

was preventing the exception handling middleware from catching exceptions. Removing it solves the problem.

answered Nov 12, 2019 at 15:30

Eutherpy

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2 Comments

WiseGuy Over a year ago

That makes sense, guessing you were trying to debug this locally with Environment Variable set to 'Development'. Probably best to wrap this in a conditional. I think it's actually like that OOTB.

dotnetdevcsharp Over a year ago

This dug me out of a whole was trying to hit brake points but couldnt cause the databasse page was getting in the way

pim3nt3l · Accepted Answer · 2019-11-12 15:59:48Z

public void Configure(IApplicationBuilder app, IHostingEnvironment env)
    {
        if (env.IsDevelopment())
        {
            app.UseDeveloperExceptionPage();
        }

        app.UseCors(ApiConstantVars.AZURE_POLICY);
        app.UseCors(ApiConstantVars.CORS_POLICY_NAME);
        app.UseAuthentication();
        app.UseMvc();

        app.Run(
               async context =>
               {
                   var handler = context.Features.Get<Microsoft.AspNetCore.Diagnostics.IExceptionHandlerFeature>();
                   if (handler != null && handler.Error != null)
                   {
                       var errorModel = new OperationResult
                       {
                           IsSuccess = false,
                       };
                        //Handle the JSON request to respond in its origin format
                       if (context.Request.ContentType.ToUpper().IndexOf("JSON") != -1)
                       {
                           context.Response.ContentType = "application/json";
                           context.Response.StatusCode = 400; //Set the Status Code to responde JSON Failed requests
                           await context.Response
                           .WriteAsync(Newtonsoft.Json.JsonConvert.SerializeObject(new { error = "Unhandled internal error", success = false, origin = context.Request.Path }))
                           .ConfigureAwait(false);
                       }
                       else
                       {
                           context.Response.StatusCode = (int)System.Net.HttpStatusCode.InternalServerError;
                           await context.Response.WriteAsync(Newtonsoft.Json.JsonConvert.SerializeObject(handler.Error.Message))
                               .ConfigureAwait(false);
                       }
                   }
                   else
                   {
                       context.Response.StatusCode = (int)System.Net.HttpStatusCode.InternalServerError;
                       await context.Response
                           .WriteAsync(Newtonsoft.Json.JsonConvert.SerializeObject(new { message = "Unhandled internal error" }))
                           .ConfigureAwait(false);
                   }
               });
    }

Muhammed Zayif · Accepted Answer · 2020-03-21 22:33:03Z

1

app.UseMiddleware<ErrorHandlingMiddleware>();

I think you should use it this way. Add this line to the Configure method

answered Mar 21, 2020 at 22:33

Muhammed Zayif

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