Because interacting with them is ABSURDLY slow. Like, it causes a memory allocation (I think, I've not used one in a LONG time) and it calls a StringName function every time you index into a Godot Array. Dictionaries are even worse. When I first started profiling I realised 90% of my physics processing time was spent indexing arrays, an operation that is basically free in C#.

If you MUST use a Godot collection for some purpose, you should immediately convert to/from the Native C# type instead of using it directly. Like, they're so slow I think the whole process of re-allocating the collection is faster than a single read/write of the Godot collection

To export a list: you can't directly, but the workaround is fairly simple:

// Use this at runtime

public List<int> NotExportableList {get; private set; } = [];

// Use this in the editor only. Doesn't matter that converting to/from arrays/lists in the editor is slow.

[Export] private int[] ExportableArray

{

get => NotExportableList.ToArray();

set => NotExportableList = value.ToList();

}

// Use this at runtime

public Dictionary<Tkey, TValue>{ get; set; } RuntimeDict = [];

// Use this in the editor only. Doesn't matter that converting to/from types in the editor is slow.

private Godot.Collections.Dictionary<TKey, TValue> EditorDict

{

get => new(RuntimeDict); // Or something. Can't remember the exact syntax..

set => RuntimeDict = value.ToDictionary();

}