If you use runtime compilcation to process .cshtml file, you might got the following error after upgrade to Asp.net core 3.0.
InvalidOperationException: The view 'Index' was not found. The following locations were searched: /Views/Home/Index.cshtml /Views/Shared/Index.cshtml
This is because of the "build and publish-time compilation of Razor file" is enabled by default, and your project file must still have the following settings:
<RazorCompileOnBuild>false</RazorCompileOnBuild>
<RazorCompileOnPublish>false</RazorCompileOnPublish>