Fundamentals > App startup article overhaul

[guardrex]

Fixes #37590
Fixes #37681
Addresses #34356

Notes

Wade ... Engineers are too busy to look at this one. Let's go ahead on our own and try to get it reviewed, updated, and merged. Remaining Copilot suggestions are either unclear or unimportant, such as its insistence that we keep the dreaded named anchors. This is ready for review! 🎉

Per our brief convo on the Fundamentals overview PR, we might need to discuss these in-repo samples further. I don't understand the concern about dropping them. IMO, the tutorials, such as the Blazor tutorials, are the best way to walk a dev through working code for base case learning scenarios. Then, advanced samples exist for difficult concepts that run under recent/current .NET versions with modern tooling and C# language versions that are easily accessible for inspection and download.

• There's repo bloat here with samples that ...
  • Either don't compile and/or target out-of-support versions of .NET.
  • Only rarely maintained with the latest best practices and new framework features and/or C# language features.
  • Never adopted the promise that they would be built by the repo build system (although that can be set up).
  • Not accessible for inspection or download by readers, only providing article snippet code.
  • Are confusing with the compiler directive demo approach.
  • Are overkill for the simple points that the text is making.
  • Bloating the repo (>189 thousand lines in 164 files).
• There's little info on what's actually happening during startup, including for the <6.0 content, so I've written out some guidance to help devs understand app startup. I'm focusing that content on >=6.0, but I can expand it back into the <6.0 part of the article if desired.
• WRT the Startup class coverage only appearing for <6.0 in spite of current support, I think that was a management decision because the PU wants devs to focus on the minimal hosting model. We have some current version content to cross-link the article's 5.0 version if a dev is upgrading an app and looking for the coverage. I left this paradigm in place for the Startup class coverage.
• ServerReady/startup performance is worth a mention, but I place a new, short section on it in the Logging overview (as an EventSource subsection) and cross-link to it there.
• IMO, the startup filters section includes an overkill example that's easily demonstrated with two cut-'n-paste snippets. I revised and hopefully improved the content. I refer readers to the middleware coverage for further details on the middleware aspects.

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Internal previews

File	Preview link
aspnetcore/fundamentals/configuration/index.md	Learn preview
aspnetcore/fundamentals/host/hosted-services.md	Learn preview
aspnetcore/fundamentals/logging/index.md	Learn preview
aspnetcore/fundamentals/startup.md	Learn preview
aspnetcore/migration/31-to-60.md	Learn preview
aspnetcore/migration/50-to-60-samples.md	Learn preview
aspnetcore/migration/50-to-60.md	Learn preview
aspnetcore/test/integration-tests.md	Learn preview
aspnetcore/test/integration-tests/includes/integration-tests7.md	Learn preview
aspnetcore/test/integration-tests/includes/integration-tests8.md	Learn preview
aspnetcore/test/integration-tests/includes/integration-tests9.md	Learn preview

Build report

[wadepickett]

@guardrex: Approved, but note my inline suggestion.

[guardrex]

@wadepickett ... Got it ... I added the versioning for the Startup class content that should only appear for >=6.0. It's the last commit ...

2a0a7cd