I can't calculate Convex hull of a point collection with a shape file.

[GangHan]

I have a point shape file in attachment. I want to calcute the convex hull polygon of the points in that file. But when I use the code as follows, It only return a blank PointCollection. Why? The shape file is right in Arcmap or ArcCatalog. I use the arcgis 9.3.

The pts is the arraylist that I read the shape file in attachment.

public ArrayList ConvexHull(ArrayList pts)
        {
            IGeometryCollection geomCol = null;
            object missing = Type.Missing;
            IPoint geom;


            geomCol = new MultipointClass();

            for (int i = 0; i < pts.Count; i++)
            {
                geom = new PointClass();
                geom.X = Convert.ToDouble(((MyPoint)pts).x);
                geom.Y = Convert.ToDouble(((MyPoint)pts).z);
                geomCol.AddGeometry(geom as IGeometry, ref missing, ref missing);
            }

            ITopologicalOperator topoOP = geomCol as ITopologicalOperator;
            if (!topoOP.IsSimple) topoOP.Simplify();
            IGeometry convexHull = topoOP.ConvexHull();
            IPointCollection ps = (convexHull as IPolygon) as IPointCollection;
            ArrayList c = new ArrayList();
            for (int i = 0; i < ps.PointCount; i++)
            {
                c.Add(ps.get_Point(i));
            }
            return c;
        }

The ps object is a blank pointCollection after run the code.

Thank you.