Files to look at

1. Program.cs line 4 .AddRazorRuntimeCompilation()
2. GlobalUsings.cs contains a global using for System.Diagnostics
3. Index.cshtml contains a line of code that requires System.Diagnostics Debugger.IsAttached

Running the project currently results in this error:

An error occurred during the compilation of a resource required to process this request. Please review the following specific error details and modify your source code appropriately.
C:\Git\GitHub\AddRazorRuntimeCompilation-Global-Usings-Example\AddRazorRuntimeCompilationGlobalUsingsExample\AddRazorRuntimeCompilationGlobalUsingsExample\Views\Home\Index.cshtml

The name 'Debugger' does not exist in the current context
+
    @if (Debugger.IsAttached)

If you add the using back to Index.cshtml it will work. Or if you remove .AddRazorRuntimeCompilation() from Program.cs The global using is not picked up when the page is compiled at runtime.