How to troubleshoot: “An error occurred while starting the application” in ASP.NET Core on IIS

The Problem

If you’ve ever seen this message when hitting your ASP.NET Core app:

“An error occurred while starting the application.  .NET Framework <version number> | Microsoft.AspNetCore.Hosting version <version number> | Microsoft Windows <version number>”

It looks a little something like this:

 

What Happened?

It basically means something really bad happened with your app.  Some things that might have gone wrong:

1. You might not have the correct .NET Core version installed on the server.
2. You might be missing DLL’s
3. Something went wrong in your Program.cs or Startup.cs before any exception handling kicked in

 

Event Viewer (probably) won’t show you anything

If you’re running on Windows and behind IIS, you might immediately go to the Event Viewer to see what happened based on your previous ASP.NET knowledge.  You’ll notice that the error is not there.  This is because Event Logging must be wired up explicitly and you’ll need to use the Microsoft.Extensions.Logging.EventLog package, and depending on the error, you might not have a chance to even catch it to log to the Event Viewer.

 

How to figure what happened (if running on IIS)

Instead of the Event Viewer, if you’re running behind IIS, we can log the request out to a file.  To do that:

1. Open your web.config
2. Change stdoutLogEnabled=true
3. Create a logs folder
   1. Unfortunately, the AspNetCoreModule doesn’t create the folder for you by default
      1. If you forget to create the logs folder, an error will be logged to the Event Viewer that says: Warning: Could not create stdoutLogFile \\?\YourPath\logs\stdout_timestamp.log, ErrorCode = -2147024893.
   2. The “stdout” part of  the value “.\logs\stdout” actually references the filename not the folder.  Which is a bit confusing.
4. Run your request again, then open the \logs\stdout_*.log file

Note – you will want to turn this off after you’re done troubleshooting, as it is a performance hit.

So your web.config’s aspNetCore element should look something like this

 <aspNetCore processPath=”.\YourProjectName.exe” stdoutLogEnabled=”true” stdoutLogFile=”.\logs\stdout” />

 

Doing this will log all the requests out to this file and when the exception occurs, it will give you the full stack trace of what happened in the \logs\stdout_*.log file

 

 

 

Hope this helps.  This happened to me a friend of mine.  A friend.  Yep.  Definitely not me.  My apps would never bomb.