Convert GameObject back to generic type

Question

I am implementing a deep object copier for Unity.

I found this great serialization/deserialization method here: https://stackoverflow.com/a/78612/3324388

However I hit a snag with MonoBehaviour objects. If the type is a GameObject, I need to use Instantiate instead of the serialization. So I've add a check:

if (typeof(T) == typeof(GameObject))
{
    GameObject clone = Instantiate(source as GameObject);
    T returnClone = clone as T;
    return returnClone;
}

I am able to cast the source as a GameObject (using as) but when I try to do it in reverse it fails with

The type parameter T cannot be used with the as parameter because it does not have a class type constraint nor a 'class' constraint.

If I try just casting it like:

if (typeof(T) == typeof(GameObject))
{
    GameObject clone = Instantiate(source as GameObject);
    T returnClone = (T)clone;
    return returnClone;
}

Cannot convert GameObject to type T

I feel I'm close but I can't quite get the casting right. Do you know what I am missing to get this to work?

If I cast the type to conform the error still persists:

In that case I get this error: [Error] The type `T' cannot be used as type parameter `T' in the generic type or method `UnityEngine.Object.Instantiate(T)'. There is no boxing or type parameter conversion from `T' to `UnityEngine.Object' — Aggressor, Jan 01 '18 at 16:55
Sounds like you need to constrain `T` whereever you are declaring it (you don’t show that) like `class MyClass where T : GameObject { ... }`. — blins, Jan 01 '18 at 17:05
Yes I had thought of that but that will restrict this to not work with mixed objects that aren't GameObjects — Aggressor, Jan 01 '18 at 17:08
I still get the cast error when I do that. I posted an image to show. — Aggressor, Jan 01 '18 at 17:13
Doesn't work, says cast is redundant and it still cant do the conversion. — Aggressor, Jan 01 '18 at 17:17
@Agressor Your `clone` object is of type GameObject, which, based on your condition, is the same as `T`. Why are you casting `source` to a `GameObject` when sending it to the `Instantiate` method? Can't you simply do `T clone = Instantiate(source)`? — IPValverde, Jan 01 '18 at 17:25
Sadly it won't cast, as mentioned above, none of that works with the errors posted above. — Aggressor, Jan 01 '18 at 17:49
I'm going to assume you've tried `Convert.ChangeType(clone, T);`? — Fredrik Schön, Jan 01 '18 at 18:06

score 5 · Accepted Answer · answered Jan 01 '18 at 18:47

Using as T in the return statement seems to do the trick. In the following test code attached to a game object in the scene I see a clone of Test and the console shows me different values for Count:

public class Test : MonoBehaviour
{
    private static bool _cloned = false;

    public static T Clone<T>(T source) where T : class 
    {
        if (typeof(T) == typeof(GameObject))
        {
            GameObject clone = Instantiate(source as GameObject);
            return clone as T;
        }
        else if (typeof(T) == typeof(PlainType))
        {
            PlainType p = new PlainType();
            // clone code
            return p as T;
        }
        return null;
    }

    public class PlainType
    {
        private static int _counter = 0;
        public int Count = ++_counter;
        public string Text = "Counter = " + _counter;
    }

    public PlainType MyPlainType = new PlainType();

    void Update ()
    {
        if (!_cloned)
        {
            _cloned = true;
            Clone(gameObject);
            PlainType plainClone = Clone(MyPlainType);
            Debug.Log("Org = " + MyPlainType.Count + " Clone = " + plainClone.Count);
        }
    }

}

This does exclude cloning any value type, but if that is not a problem it's definitely cleaner and prettier than the intermediate cast to `object`. @Aggressor — InBetween, Jan 02 '18 at 08:39

InBetween · Answer 2 · 2018-01-02T08:54:38.640

4

It’s not pretty but you can force the compiler doing a previous reference conversion to object:

 public static T Clone<T>(T source)
 {
      if (source is GameObject)
      {
          return (T)(object)Instantiate((GameObject)(object)source);  
      }
      else ...
  }

Yes, it is a bit of hack but sometimes you can’t avoid it. As a general rule, when you start mixing generics with runtime type checks things tend to get messy, a sure sign that you probably shouldn’t be using generics to begin with. Sometimes though, it can be justified but ugly code tends to crop up.

edited Jan 02 '18 at 08:54

answered Jan 01 '18 at 18:27

InBetween

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Works, but I prefer the `return p as T;` solution in Kay's answer. – Draco18s no longer trusts SE Jan 01 '18 at 19:35
I'm trying to figure out how you figured this out but I draw a blank. Kudos. This works. – Aggressor Jan 02 '18 at 03:20
@Draco18s oops sorry, forgot to remove the `class` constraint. This works for any type. – InBetween Jan 02 '18 at 07:11

Convert GameObject back to generic type

2 Answers2