Question

I am new in C#. I am from Java world. So I am confused with the following code:

    class A
    {
        private PointF point;

        public A(PointF point)
        {
            this.point = point;
        }

        public PointF Position
        {
            get { return point; }
        }
    }

I want to change X-coordinate of the position property, so I perform:

        A a = new A(new PointF(1,2));
        PointF p = a.Position;
        p.X = 100;
        Console.WriteLine(a.Position.X); // <--- I have 1 here!

I wonder why the output is NOT 100? As I have understand I have received reference on private field with Position property. Am I right?

Can I change property without adding set-property and propagating Position with new PointF object?

Answer 1

No; PointF is a struct, so it has copy semantics and is not an object; as soon as you’ve obtained it – it is a separate and isolated copy (unless you’re using something like ref/out, which is … more subtle). The struct is actually copied multiple times in your example.

By the way, a consequence of this is that it is actually a very bad idea to have mutable structs – so in most cases you should avoid the scenario where you can say:

p.X = 100;

since that causes more confusion than it helps. In particular the following is completely invalid:

a.Position.X = 100; // won't even compile

(here the compile is spotting that you are changing a copy of the struct that only exists during the mutation itself, which means your change goes nowhere, and is almost certainly a bug)