The WPF markup compiler generates the program entry point from the
ApplicationDefinition item, so an exported project that lists App.xaml
as a Page has no Main at all and fails to build with CS5001. Both the
UI and ilspycmd already resolve the BAML root's partial class, which
makes deriving Application from System.Windows.Application the natural
signal. The module additionally has to have an entry point of its own:
a library that merely contains an Application subclass would otherwise
have MSBuild generate a Main into it.
Assisted-by: Claude:claude-opus-5[1m]:Claude Code
Siegfried Pammer