c# parameter passing in ref with class field

Question

I want to swap the field in the ConvexHull class just like swap( points[0], points[1] ).

How do I have to do?

public class ConvexHull
{
    List<Point> points;

    public void run ()
    {
        Point.swap ( ref points[ 0 ], ref points[ 1 ] );  //Error!!
    }
}

public class Point
{
    private double x, y;

    Point () { x = y = 0; }
    public static void swap(ref Point a, ref Point b) {
        Point c = a;
        a = b;
        b = c;
    }
}

It seems an odd design to have the Point class do the swap. Why don't you have the swap method in the ConvexHull class and pass the int index values of the two positions you want to swap? — hatchet - done with SOverflow, Apr 16 '15 at 00:50
Also seems odd that users are unable to read or write the `x` and `y` values of a `Point`. I mean, what's the `Point`? — Rufus L, Apr 16 '15 at 00:52
The error that code will generate is the `argument is not classified as a variable` compile error. — hatchet - done with SOverflow, Apr 16 '15 at 00:54

score 1 · Accepted Answer · answered Apr 16 '15 at 01:16

When you index an element of List<T> you are actually accessing the this indexer, which is a kind of property (i.e. has getter and setter methods). You can only pass variables as ref or out, not properties.

In your scenario, perhaps you want something more like this:

public class ConvexHull
{
    List<Point> points;

    public void run ()
    {
        swap(0, 1);  //No error!!
    }

    private void swap(int i, int j)
    {
        Point point = points[i];

        points[i] = points[j];
        points[j] = point;
    }
}

A more general solution might look like this:

public class ConvexHull
{
    List<Point> points;

    public void run ()
    {
        points.SwapElements(0, 1);
    }
}

static class Extensions
{
    public static void SwapElements<T>(this List<T> list, int index1, int index2)
    {
        T t = list[index1];

        list[index1] = list[index2];
        list[index2] = t;
    }
}

In either case, the correct approach is to provide the code that is actually swapping values with access to the List<T> object itself, so that it can access the indexer property to accomplish the swap.

score 0 · Answer 2 · answered Apr 16 '15 at 01:19

Throw pretty much all of that away. You can't pass properties or list objects by ref. I notice there is nothing initially populating those points. Populate your List of Points, then call a function in your ConvexHull class to SwapPoints(int point1idx, int point2idx) and write the code there to do the swap.

On the Point class, expose X and Y, and drop the swap routine from there as it will never work.

c# parameter passing in ref with class field

2 Answers2