Holger's Blog : ASP.NET

Identify ASP.Net Core startup errors

Holger Vetter — Wed, 30 Nov 2016 15:31:00 GMT

Maybe you know this great and informative error message "An error occurred while starting the application." from asp.net core when there was an exception during the startup.

To identify and understand what happened, you must extend the Main method with the following things:

UseSetting("detailedErrors", "true")
CaptureStartupErrors(true)

After this is done, you will see a nice and detailed error page like this:

In my case the startup error was a missing json file which the application tried to load on startup.

ASP.NET Core returns StatusCode:500

Holger Vetter — Thu, 10 Nov 2016 12:56:24 GMT

I ported an MVC WEB API project to ASP.NET Core and now the Service just returned StatusCode: 500 every time I called it.

Visual Studio doesn't give me any information or Exception to indicate what happened.

To find the error I disabled "Enable just my Code" in the visual studio settings.

Now the visual studio showed this exception from the framework, when I call the service:
"Multiple constructors accepting all given argument types have been found in type "…". There should only be one applicable constructor".

This error indicates that we have multiple constructors, which accepts same argument types. But this wasn't the case, I just removed the empty constructor and it worked fine.

One was the empty constructor I had before in my ASP.NET WEB API application and the other one was for ASP.NET Core.
I just removed the empty one and it worked fine J

Recap: Don't use multiple constructors when injection IOptions.

ASP.NET MVC WEB API Impersonation Thread issue

Holger Vetter — Tue, 20 Sep 2016 14:53:00 GMT

In an ASP.NET MVC WEB API application I implemented Windows Authentication with custom roles by using an implementation of IIdentity.
In my setup the user gets authenticated by NTLM, then we have the user name. Now there is a part in the OWIN Pipeline which gets the roles for this user and caches them.

When the user sends a request to the server it will go trough the “Application_OnPostAuthenticateRequest“method in the Global.asax.cs, this method will then get the User from the current thread (which is the authenticated user) and then create a new object of our custom implementation of IIdentity. After this we map some things like the UserName and get the correct roles for this user and append them.

The important part in the “Application_OnPostAuthenticateRequest” method is to set the modified Thread.CurrentPrincipal back to the HttpContext.Current.User. If you don’t do this, you possible lose our custom Identity object next time ASP.NET maps/sets the HttpContext.Current.User to Thread.CurrentPrincipal (Thread.CurrentPrincipal gets overriden by HttpContext.Current.User).

In my tests the ManagedThreadId in OnPostRequest method and the actual service operations where different, but not everytime. When I attach a debugger it worked. This leaded me to a threading problem. In the OnPostRequest method I set the Thread.CurrentUser and in my service operation I was in a different thread, that’s why the custom roles disappeared.

Maybe you have the some problem and this helps you :)