Using C# object as a metatable

[Charanor]

Tl;dr

Assigning the ScriptableDoor as metatable index results in LuaRuntimeException: bad argument #1 to 'index' (ScriptableDoor expected, got Table). This is because the generated class does this in its own metatable indexer context.GetArgument<global::ScriptableDoor>(0):

var csObject = new ScriptableDoor(); // Has [LuaObject] attribute with [LuaMember] members
var table = new LuaTable();
table.Metatable = new LuaTable()
{
    [Lua.Runtime.Metamethods.Index] = csObject
};
state.Environment["obj"] = table;

Then in lua:

-- Exception
obj:OnOpen(nil);

---

I'm trying to allow users to extend C# objects with Lua code to allow for more object-oriented scripting. For example a scriptable C# object might look like this:

[LuaObject]
public class ScriptableDoor
{
    /* Lua state and stuff saved as well */

    public ScriptableDoor(LuaTable scriptClass)
    {
        // Create LuaObject below, set states, metatables, etc.
    }

    public LuaTable LuaObject { get; }
    public bool IsOpen { get; private set; }

    [LuaMember]
    public void OnCreated()
    {
        // NOOP
    }

    [LuaMember]
    public void OnOpen(Entity entity)
    {
        // Open the door
        this.IsOpen = true
    }

    [LuaMember]
    public void OnClose(Entity entity)
    {
        // Close the door
        this.IsOpen = false
    }
}

Then one could create a script for such a door like this:

-- object_trap_door.lua
object_trap_door = object_trap_door or {}

function object_trap_door:OnCreated()
    self.damage = 3
end

function object_trap_door:OnOpen(entity)
    self.Base:OnOpen(entity)
    entity:DealDamage(self.damage)
end

The idea is that it can then be used like this:

local instance = CreateInstance(object_trap_door)
instance:OnOpen(player) -- Uses OnOpen from the script definition
instance:OnClose(player) -- Uses OnClose from the __index (the C# class).

CreateInstance would look something like:

var state = LuaState.Create();
await state.DoFileAsync("object_trap_door.lua");
var scriptClass = state.Environment["object_trap_door"].Read<LuaTable>();

var csInstance = new ScriptableDoor(scriptClass);
var luaInstance = csInstance.LuaObject;
luaInstance.Metatable = new LuaTable()
{
    [Lua.Runtime.Metamethods.Index] = csInstance,
};
await luaInstance[nameof(ScriptableDoor.OnCreated)].Read<LuaFunction>().InvokeAsync(state, [luaInstance]);

return luaInstance;

However this doesn't work. Assigning the ScriptableDoor as metatable index results in LuaRuntimeException: bad argument #1 to 'index' (ScriptableDoor expected, got Table). This is because the generated class does this in its own metatable indexer context.GetArgument<global::ScriptableDoor>(0).

This could be fixed by creating a custom metatable indexer of course, but outside of reflection there is no convenient way to do this.