Holger's Blog : Core.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.

.NET Core json settings and environment variable

Holger Vetter — Wed, 19 Oct 2016 15:19:52 GMT

Today we deployed .NET Core application on a test machine and then configured the application as environmental variable in the system.

When we started the command line application, it couldn't find 'settings.json' file for reading the configuration.

This happens because the application looks for it on the relative path where the command was executed.
For example application is in C:\Temp\tool.exe and you run enter 'tool' while in 'C:\Users\hvetter', the application will start but will not found your settings file because it searches it in 'C:\Users\hvetter' folder instead of 'C:\Temp\'.

This is the code we used to load the settings file and the output in the Commandline.

SETTINGS_FILE_NAME is a constant and equals 'settings.json'.

It searched 'settings.json' file in relative wrong folder.

I changed the code to this for testing and added the output to show what happened.

Recap: Don't use relative paths in .Net Core for loading your settings :)