C#: Generate instance types for singletons

[raulsntos]

Singleton classes in C# are currently exposed as static classes. In some cases these classes are inherited (e.g.: PhysicsServer2DExtension derives from the singleton class PhysicsServer2D).

We currently ignore these classes and don't generate them, because C# doesn't allow inheriting static classes. In order to fix this, with this PR C# now generates additional Instance classes for every singleton. E.g.:

// Good old static singleton class. Still generated.
public static class PhysicsServer2D { ... }

// New singleton instance class.
public class PhysicsServer2DInstance { ... }

// Now we can generate PhysicsServer2DExtension, deriving from the instance class.
public class PhysicsServer2DExtension : PhysicsServer2DInstance { ... }

• Constants and enums remain in the static singleton class (e.g.: Input.MouseModeEnum).
• Paves the way to eventually remove the static singleton classes¹.
  • Previous attempt: Generate C# singletons as non-static classes #59208
• Considerably simplifies the implementation of C#: Add WeakEvent to store subscriptions of events #73730 for the singleton classes, since it can reuse the existing system used by instance classes to raise the events.

Footnotes

1. This would break compatibility and won't happen in 4.x, but in 5.0 we could remove the static classes, rename the instance classes to remove the Instance suffix and create a Singletons static class that can be used with global using. Similar to how singletons are exposed in GDScript using @GlobalScope. See https://github.com/godotengine/godot/pull/59208#issuecomment-1069583895. ↩

[YuriSizov]

Does this help with resolving compatibility issues from #75694 ?

[raulsntos]

As I understand it, #75694 changes EditorInterface to be a singleton. This means in C# the EditorInterface becomes static which breaks compat. This PR doesn't help with that, the EditorInterface would still be generated as a static class after #75694.

However, with this PR we generate an additional EditorInterfaceInstance class which is not static and would be essentially the same class that existed in 4.1. So I could hardcode this class so it doesn't add the Instance suffix and to avoid generating the static class, then it should avoid breaking compat.

[YuriSizov]

However, with this PR we generate an additional EditorInterfaceInstance class which is not static and would be essentially the same class that existed in 4.1. So I could hardcode this class so it doesn't add the Instance suffix and to avoid generating the static class, then it should avoid breaking compat.

That sounds like a great idea, if it's possible. I think if we are going to merge this PR, we should merge it first, then you can suggest me how to update mine to make address the issue.

[raulsntos]

I've added a commit with the some of the changes suggested by @neikeq in #75694 (comment). This means that now EditorInterface will still be generated as a non-static class, but your PR should not remove the get_editor_interface() methods to avoid breaking compat entirely.

[RedworkDE]

Some more minor suggestions, but I (still) cannot think of a fundamentally better design for this.

[akien-mga]

Thanks!