How to assign a value in other classes?

Question

I want to create a aminator class. But it can't modify the field value in other classes.

Here is my simplified animator class:

public class PointMover
{
    Point point;
    public void Set(ref Point p)
    {
        point = p;
    }

    public void Move(int dX)
    {
        point.X += dX;  // The point.X is modified here.
    }
}

and my main class:

public partial class Form1 : Form
{
    PointMover pointMover = new PointMover();
    Point point = new Point(0, 0);

    private void Form1_Load(object sender, EventArgs e)
    {
        pointMover.Set(ref point);
        pointMover.Move(10); // But point.X is NOT modified here.
        this.Close();
    }
}

Here is my problem. Is there anyone knows how to fix it? I'll be appreciate it.

Answer 1

Point is a struct (i.e. value type). You pass it by reference, but you then create copy of point instance in constructor of PointMover by assigning it to point field:

public void Set(ref Point p)
{
    point = p; // here you create copy of passed point
}

Thus modifications of point don't affect p (because they represent different instances of structure).

Note: If Point would be a reference type (i.e. class) then this assignment would copy a reference, and both variables would have references to same instance in heap.

---

In order to fix this behavior, you need to modify point which was passed by reference without creating copy. E.g.

public static void Move(ref Point point, int dX)
{
    point.X += dX; 
}

Usage:

PointMover.Move(ref point, 20);

Or you can simply use Point.Offset(int dx, int dy) method.