Startup class in a library is not working

[iiibbbmmm]

Describe the bug

Startup class in a library is not working

To Reproduce

1. Create a new WebApplication with ASP.NET Core Web API presets (no HTTPS/Docker/OpenAPI).
2. Create a new ClassLibrary(.NET Core) in the same solution.
3. Reference ClassLibrary by WebApplication.
4. Shift+Drag the Startup.cs from WebApplication to ClassLibrary.
5. Put these in ClassLibrary.csproj.

  <ItemGroup>
    <FrameworkReference Include="Microsoft.AspNetCore.App" />
  </ItemGroup>

6. Press F5.

Exceptions (if any)

HTTP ERROR 404

Further technical details

• ASP.NET Core version
  5.0
• Include the output of dotnet --info

info: Microsoft.Hosting.Lifetime[0]
      Now listening on: http://localhost:5000
info: Microsoft.Hosting.Lifetime[0]
      Application started. Press Ctrl+C to shut down.
info: Microsoft.Hosting.Lifetime[0]
      Hosting environment: Development
info: Microsoft.Hosting.Lifetime[0]
      Content root path: C:\dev\projects\aspnetcore-test\WebApplication1

• The IDE (VS / VS Code/ VS4Mac) you're running on, and its version
  VS 2019 16.8.3

[Tratcher]

Duplicate of #7315.

[shirhatti]

Closing as dupe