Microsoft.AspNetCore.Hosting throws Startup assembly StartupBootstrapper failed to execute

I see these errors logged on Azure (App Service) on every restart of the App Service:

Main message:

Startup assembly StartupBootstrapper failed to execute. See the inner exception for more details. Could not load file or assembly 'StartupBootstrapper, Culture=neutral, PublicKeyToken=null'. The system cannot find the file specified. 

Stack trace:

System.InvalidOperationException:
System.IO.FileNotFoundException:
   at System.Reflection.RuntimeAssembly.nLoad (System.Private.CoreLib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e)
   at System.Reflection.RuntimeAssembly.InternalLoadAssemblyName (System.Private.CoreLib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e)
   at System.Reflection.Assembly.Load (System.Private.CoreLib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e)
   at System.Reflection.Assembly.Load (System.Private.CoreLib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e)
   at Microsoft.AspNetCore.Hosting.GenericWebHostBuilder.ExecuteHostingStartups (Microsoft.AspNetCore.Hosting, Version=3.0.0.0, Culture=neutral, PublicKeyToken=adb9793829ddae60)

Let me know what additional data I can collect for you that may help in fixing this issue.

[davidfowl]

This happens when the IHostingStartup in site extensions fail to load. StartupBootstrapper is an application insights type see https://github.com/Microsoft/ApplicationInsights-Home/issues/231.

[davidfowl]

BTW is your application self contained?

@davidfowl :

I tried reading https://github.com/microsoft/ApplicationInsights-Home/issues/231 and the attached issue #5919. It is unclear what exactly resolved the issue -- and if in fact the issue was resolved in those cases.

Yes, my application is self-contained, here's the config:

Done (and desired) by me, manually:

• Azure App Service, Windows environment, Code deployment
• Code built on .NET Core 3.0 Preview, C# 8.0 Preview (cutting bleeding edge!) using Azure DevOps CI pipeline from a GitHub repo.
• Application Insights installed into app via Visual Studio's Connected Services > Application Insights page. It shows "100% configured".
• Application Insights is enabled in "full" mode (everything except SQL collection is ON) in Azure Portal.

I didn't want to go into Azure Portal and keep updating extensions there, so I haven't installed the .NET Core 3.0 Site Extension -- this is why I have it deployed in SCD mode.

[davidfowl]

Unfortunately right now, the site extensions do not work with self contained deployments. You'll need to add the Application Insights SDK manually to get it working.

I didn't want to go into Azure Portal and keep updating extensions there, so I haven't installed the .NET Core 3.0 Site Extension -- this is why I have it deployed in SCD mode.

I think you'd need to remove the site extension to prevent it from throwing at startup.

The only Site Extension present on this app service (as per App Service > Extensions blade on the portal is ASP.NET Core Logging Extensions of version 2.2.0. And, I have not installed this myself, it is something Azure does.

Could you clarify what you mean by "You'll need to add the Application Insights SDK manually" ? I have installed it from within the Visual Studio IDE.

[davidfowl]

OK I spent some time looking into this and you can't get rid of this exception today if the application is self contained. The feature being used to inject application insights into the application always fails on startup (because of a limitation in the current design).

The only way to get rid of it today is to do a framework dependent deployment by either waiting for 3.0 to be installed on app service or by using the ASP.NET Core runtime site extension.

I'll keep this issue open and assign it to myself as it's something we need to solve.

Thanks @davidfowl.

Strange thing is, the Application Insights seems to be working fine. I see the traffic, exceptions and everything else as I normally should. This means App Insights is still being loaded?

[davidfowl]

Strange thing is, the Application Insights seems to be working fine. I see the traffic, exceptions and everything else as I normally should. This means App Insights is still being loaded?

Yes, it works fine but it'll still try to load some other pieces of code at startup and always fail.

Tangential question - Is there a way to simulate how an self-contained deployment would work in a local (laptop/PC) environment?

[davidfowl]

What do you mean exactly? You can self contain publish and then run the published application locally? Is there something more you were looking for?

Let's say you need to troubleshoot a problem (like the topic of this assembly load). You'd need an environment that mimics it right? So how would you mimic an SCD environment?

When I publish and run an SCD application locally, the runtime is already present globally because of my primary .NET Core installation.

[davidfowl]

Its doesnt matter if the runtime is already present globally it won't be used. The application produced has no ties to the globally installed runtime.

[analogrelay]

I'll keep this issue open and assign it to myself as it's something we need to solve.

What's the action you want to take here @davidfowl ?