[C#] Godot can't find C# mainloop

[Hromon]

Godot version

4.0 beta 4 and beta 7

System information

Windows 10

Issue description

Godot can't find the C# mainloop, but if create a GDScript variant, then everything works

Steps to reproduce

Create C# script and inherit it from the mainloop and specify it in the settings.

Minimal reproduction project

TestProject.zip

[raulsntos]

It seems Godot looks for the type in ClassDB or global scripts. The .NET module currently supports neither way to register your type.

C# is currently implemented as a ScriptLanguage and therefore doesn't register C# types in ClassDB, in the future we want to allow implementing GDExtensions with C# which would register types in ClassDB.

For ScriptLanguages, such as GDScript, the type can be provided using global scripts. C# doesn't currently support this but there is a PR that adds support for this: #63126.