Why can I modify transform.position but not transform.position.y?

Unity Engine
1

I am trying to write this:

        transform.position = distanceBetweenCamAndPlayer + player.transform.position;
        transform.position.y = cameraY;

the first line works fine, but the second one displays the error: “cannot modify the return value of transform.position because it is not a variable”.
How come I can change the transform.position with a vector3 but I can’t change only the Y component?

2

Because of value types!

A Vector3 is a value type, which means that it’s passed by value. So whenever you grab a Vector3 from somewhere, you’re grabbing a copy:

Vector3 a = new Vector3(1, 2, 3);
Vector3 b = a;
b.x = 15;
Debug.Log(a.x); //still 1!

Since Transform.position is implemented as a getter, you’re getting a copy whenever you’re retrieving it. So when you do:

transform.position.x = ...

You’re trying to set the x-value on a copy of the transform’s position. The compiler disallows it, since you’d otherwise have code that does nothing that looks like it does something.

You’ll either have to get a copy, modify that, and pass it back in:

var pos = transform.position;
pos.y = cameraY;
transform.position = pos;

Or make an extension method for Transform that allows you to do what you want:

public static class TransformExtensions {
    public static void SetYPos(this Transform t, float newYPos) {
        var pos = t.position;
        pos.y = newYPos;
        t.position = pos;
    }
}

//your code above can now be:

transform.position = distanceBetweenCamAndPlayer + player.transform.position;
transform.SetYPos(cameraY);
6 Likes
3

I used to assign Vector.3 parameters of other classes so I think that example doesn’t quite fit. If position would not be a property then transform.position.x = 2 will not create a new position but assign x directly.

So getter is the reason for that here, but thanks for that explanation.

4

Plz,read about difference between class and structure

5

You can change the value, but you have to change all of them because it’s a vector3.
transform.position = new Vector3(transform.position.x, cameraY,transform.position.z);

6

Take a look at the source code for Transform:

public Vector3 position
{
   get
   {
     Vector3 result;
     this.INTERNAL_get_position(out result);
     return result;
   } 
  set
  {
     this.INTERNAL_set_position(ref value);
  }
}

transform.position gives you a copy of the position. Writing to transform.position.x would do nothing useful.

1 Like