Fix C# main screen plugin example and document C# EditorInterface singleton

[easterNday]

Correct the syntax errors in the plugin development section, specifically by modifying EditorInterface to EditorInterface.Singleton in the C# portion. It is imperative to utilise the singleton pattern here to ensure the plugin successfully compiles.

• Docteam edit, closes: Plugins docs on Dotnet C# EditorInterface to get singleton is wrong and misleading #9626*

[AThousandShips]

Are you sure this is needed? See here

[easterNday]

The EditorInterface isn't a static class, and in the course of developing plugins for Godot's main screen, it's essential to utilise the singleton instance of EditorInterface; I fancy this to be a prerequisite. Indeed, attempts to proceed without incorporating this singleton would meet with compilation obstacles, rendering the process unfeasible.

[paulloz]

Are you sure this is needed?

Yes, it is totally needed 🙂

[AThousandShips]

Then this singleton is an exception, should probably be added to the documentation

[easterNday]

I'm not entirely sure if this fully meets your requirements, but I have added a specific exception explanation to that section.

[paulloz]

I think if we want to make a table listing everywhere .Singleton is necessary, we should do it exhaustively (could be done in its own PR). IMHO, an incomplete list is as bad as the current status quo.
FWIW, examples in the class reference are already up-to-date. E.g.

godot-docs/classes/class_editorplugin.rst

Line 860 in 83a98d8

return EditorInterface.Singleton.GetEditorTheme().GetIcon("Node", "EditorIcons");

[easterNday]

I think if we want to make a table listing everywhere .Singleton is necessary, we should do it exhaustively (could be done in its own PR). IMHO, an incomplete list is as bad as the current status quo.

You're right, I will follow your advice and complete this list accordingly.

[raulsntos]

EditorInterface is the only one:

https://github.com/godotengine/godot/blob/daa81bbb7d1c6d75d1711595604178ee62a5801d/modules/mono/editor/bindings_generator.cpp#L4801-L4803

This is because historically this class wasn't registered as a singleton, so when it became one in godotengine/godot#75694 we couldn't make it static without breaking compat.

All the other singletons are static classes, although they also have a Singleton property. In this case using the static class or the instance makes no difference.

[tetrapod00]

The current state was also verbally approved by raulsntos in the RC: https://chat.godotengine.org/channel/dotnet?msg=mJnHFdxJM75PhnRWN

Since the author turned off edits by maintainers, I am merging the current acceptable state and will make a followup PR for the wording adjustments.

[tetrapod00]

Thank you, and congrats on your first merged contribution to the Godot documentation!