in c# code why below copy didn't work as reference copy?

Question

Below c# code I run in Visual Studio 2019 Mac, I am a little surprise for the result:

using System;

namespace Test
{
    public struct Point
    {
        public int x;
        private int y;
        public Point(int x, int y)
        {
            this.x = x;
            this.y = y;
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            Point p1 = new Point(100, 100);
            Point p2;
            p2 = p1;
            p1.x = 200;
            Console.WriteLine("p1.x is {0},p2.x is {1} ", p1.x, p2.x);
            // I think here should Output: p1.x is 200, p2.x is 200
            // But the actual output is: p1.x is 200, p2.x is 100, why? is it a reference copy?
            // p1 and p2 should share the same reference, right?
        }

    }
}

Actually when I read C# instruction, it explained that such code should output: p1.x is 200,p2.x is 200 because p2 and p1 share the same pointer to point to ONE address in heap, right? while when I try to test above code in VS2019 Mac. it's output is: p1.x is 200,p2.x is 100 which confused me so much? Is it a shallow copy or deep copy? Can somebody please explain why p2.x is still 100, when p1.x already changed to 200? Thanks a lot.

score 2 · Accepted Answer · answered Jul 20 '19 at 16:42

2

Your Point is a struct. When you assigned p2 to p1, p2 became a copy. I think you might want to read up a bit more on value types versus reference types.

answered Jul 20 '19 at 16:42

Moo-Juice

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score 1 · Answer 2 · answered Jul 20 '19 at 16:57

Structs are value types in C# which means that they will assign by value. That means that when you assign p1 to p2, it is copying the values instead of the memory location. If you want this to have the behavior that you are looking for, make your point struct a class. Classes are reference types.

in c# code why below copy didn't work as reference copy?

2 Answers2