C#

class ProjectDoc
{
         // there is a panel in the form //
         private System.Windows.Forms.Panel panelProjectDoc;
         this.panelProjectDoc = new System.Windows.Forms.Panel();
         this.Controls.Add(this.panelProjectDoc);
   
        public Point myfunc(Point dropPoint)
        {
            int X_snap = 0;
            int Y_snap = 0;
            //  .... do something .... //
            return new Point(X_snap, Y_snap);
        }

        private void Filter_Click(object sender, EventArgs e)
        {
            BuildBlock buildBlock = new BuildBlock();
            this.panelProjectDoc.Controls.Add(buildBlock);
            buildBlock.Location = myfunc(new Point(900, 400));  // This works OK //
        }
}

class BuildBlock : UserControl
{
        private void BuildBlock_MouseUp(object sender, MouseEventArgs e)
        {
            Point p = new Point(0, 0);
            Point q = new Point(0, 0);
            p = this.Location;
            q = (Parent as ProjectDoc).myfunc(p); // ----- Run Time ERROR on this line ----//
            this.Location = q;
        }
}