Solving Error - "An item with the same key has already been added when returning 404"

Question

I have my custom error page handler defined like this:

if (env.IsDevelopment())
{
    app.UseDeveloperExceptionPage();
    app.UseBrowserLink();
}
else
{
    app.UseExceptionHandler("/Error");
}

app.UseStatusCodePagesWithReExecute("/Error/{0}");

app.UseMvc(routes =>
{
    ...
}

Then in error controller:

public class ErrorController : Controller
{
    public IActionResult Index(int errorCode)
    {
        switch (errorCode)
        {
            case 404:
                return View($"~/Views/Error/{errorCode}.cshtml");
        }

        return View(errorCode);
    }
}

When in production mode everything works fine, but when on develop, there are 2 scenarios:

1. I'm hitting 404 page "just like that" and it's showing my custom 404 page (which is correct)

2. I'm forcing 404 from action eg.

   if(nothingFound)
   {
       return NotFound();
   }
   

   and then it's entering ErrorController action, and even the view (404.cshtml), but after that it prints whole debug titled:

ArgumentException: An item with the same key has already been added. Key: System.Object

Is it something normal when using app.UseDeveloperExceptionPage(), or do I have something miss-configured?

UPDATE

Stack trace:

System.ArgumentException: An item with the same key has already been added. Key: System.Object
   at System.ThrowHelper.ThrowAddingDuplicateWithKeyArgumentException(Object key)
   at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.<InvokeNextResourceFilter>d__22.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.Rethrow(ResourceExecutedContext context)
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.<InvokeAsync>d__20.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.AspNetCore.Builder.RouterMiddleware.<Invoke>d__4.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.AspNetCore.Authentication.AuthenticationMiddleware`1.<Invoke>d__18.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at Microsoft.AspNetCore.Authentication.AuthenticationMiddleware`1.<Invoke>d__18.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.AspNetCore.Builder.StatusCodePagesExtensions.<>c__DisplayClass6_0.<<UseStatusCodePagesWithReExecute>b__0>d.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.AspNetCore.Diagnostics.StatusCodePagesMiddleware.<Invoke>d__3.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.AspNetCore.Localization.RequestLocalizationMiddleware.<Invoke>d__4.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.VisualStudio.Web.BrowserLink.Runtime.BrowserLinkMiddleware.<ExecuteWithFilter>d__7.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.<Invoke>d__7.MoveNext()

Answer 1

I was having similar issue. I found the issue later that it was related to the order of using middleware. I was using middle ware to write response headers like X-Content-Type-Options and other headers. I just moved this above the app.UseStatusCodePagesWithReExecute and it start working correctly. So my current code is like:

app.Use(async (context, next) =>
        {
            context.Response.Headers.Add("X-Content-Type-Options", "nosniff");
            context.Response.Headers.Add("X-Xss-Protection", "1");
            context.Response.Headers.Add("X-Frame-Options", "SAMEORIGIN");
            await next();
        });

        if (env.IsDevelopment())
        {
            app.UseDeveloperExceptionPage();
            app.UseDatabaseErrorPage();
            app.UseBrowserLink();
        }
        else
        {
            app.UseStatusCodePagesWithReExecute("/Home/Error/{0}");
        }

Hope this can help any one having similar issue like me.