Why are changes in one object applied to all objects in a list?

Question

I've got two List:

1. BasicModels: As Content has to be loaded at the start of the game I instanciate and load all models into this list.

2. ModelsOnScreen: Every time when a Model is added to the screen I take the corresponding object out of BasicModels and put it into ModelsOnScreen. At the same time I write the ID of the Model at it's location in ModelsOnScreen.

So the problem is though I change the Id at a specific place in the list, changes are applied to all models in ModelsOnScreen.

 for (int i = 0; i < _game.Models.Length; i++)
        {
            if (assetName == _game.Models[i].name.ToString())
            {
                _game.modelsOnScreen.Add(_game.Models[i]);
                _game.modelsOnScreen[_game.modelsOnScreen.Count - 1].ID = nameCounter;
            }
        }

What could be the problem? My theory is that the Models I put into ModelsOnScreen are only references, no separate instances. If that is so, how could I solve this problem?

Any advice is welcome,

greets BC++

Answer 1

Yes, your suspicion is correct — the .Add() method does not make a copy of the model, so when you change _game.modelsOnScreen[_game.modelsOnScreen.Count - 1] you are also changing _game.Models[i] — they refer to the same object. It's rare for a function to make a copy of an object.

If you want many different IDs associated with the same Model, then make a new class/struct that stores an ID and a model, and create a new instance of that for each ID.

Answer 2

As far as I can tell, nameCounter is being assigned to the ID property of every single model added to modelOnScreen. Look closely at the loop and you'll see that your code is equivalent to:

for (int i = 0; i < _game.Models.Length; i++)
{
    var model = _game.Models[i];
    if (assetName == model.name.ToString())
    {
        _game.modelsOnScreen.Add(model);
        model.ID = nameCounter;
    }
}

If you only want to modify the last model added (I presume that's your intention?) move that assignment out of the loop and only do it once every model has been loaded. And yes, models are probably references, if you are using classes for them. Value types in C# are structs, enumerations and primitive types.