Question

I have a custom class CurvePoint which I define a set of data items to draw on the screen using DrawCurve

I written a cast routine to convert CurvePoint to Point, but I am getting the error
At least one element in the source array could not be cast down to the destination array type. when I try to use the .ToArray method in an Arraylist

I can cast the objects fine using the code:


Point f = new CurvePoint(.5F, .5F, new Rectangle(0, 0, 10, 10));


but if fails when using


Point[] Plots=(Point[])_Data.ToArray(typeof(Point));


(where _Data is an ArrayList which is populated with 5 CurvePoint objects)

Here is the full code:

public partial class Chart : UserControl
{
    ArrayList _Data;
    public Chart()
    {
        InitializeComponent();
        _Data = new ArrayList();
        _Data.Add(new CurvePoint(0f, 0f,this.ClientRectangle));
        _Data.Add(new CurvePoint(1f, 1f, this.ClientRectangle));
        _Data.Add(new CurvePoint(.25f, .75f, this.ClientRectangle));
        _Data.Add(new CurvePoint(.75f, .25f, this.ClientRectangle));
        _Data.Add(new CurvePoint(.5f, .6f, this.ClientRectangle));
    }
    private void Chart_Paint(object sender, PaintEventArgs e)
    {
        _Data.Sort();
        e.Graphics.FillEllipse(new SolidBrush(Color.Red),this.ClientRectangle);
        Point[] Plots=(Point[])_Data.ToArray(typeof(Point));
        e.Graphics.DrawCurve(new Pen(new SolidBrush(Color.Black)), Plots);
    }
}
public class CurvePoint : IComparable
{
    public float PlotX;
    public float PlotY;
    public Rectangle BoundingBox;
   public CurvePoint(float x, float y,Rectangle rect)
    {
        PlotX = x; PlotY = y;
        BoundingBox = rect;
    }

    public int CompareTo(object obj)
    {
        if (obj is CurvePoint)
        {
            CurvePoint cp = (CurvePoint)obj;
            return PlotX.CompareTo(cp.PlotX);
        }
        else
        { throw new ArgumentException("Object is not a CurvePoint."); }
    }
    public static implicit operator Point(CurvePoint x)
    {
        return new Point((int)(x.PlotX * x.BoundingBox.Width), (int)(x.PlotY * x.BoundingBox.Height));
    }
   public static implicit operator string(CurvePoint  x)
    {
        return x.ToString();
    }
   public override string ToString()
    {
        return "X=" + PlotX.ToString("0.0%") + " Y" + PlotY.ToString("0.0%");
    }

}

Can anyone sujest how to fix the code?

Answer 1

First, I would change your ArrayList to a strongly typed generic list, List<CurvePoint>. Then, to get your Point array, you can perform this code.

Point[] Plots = _Data.Select(obj => (Point)obj).ToArray();

If you leave it as an ArrayList, you can still do it using this code:

Point[] Plots = (from CurvePoint obj in _Data select (Point)obj).ToArray();
// or
Point[] Plots = _Data.Cast<CurvePoint>().Select(obj => (Point)obj).ToArray();

Edit: Finally, if you’re stuck with ArrayList and you do not have LINQ, you can do this the “long” way.

Point[] Plots = new Point[_Data.Count];
for (int i = 0; i < _Data.Count; i++)
{
    Plots[i] = (Point)(CurvePoint)_Data[i];
}