You need to call the Windows API SendMessage() function.

        //using System.Runtime.InteropServices;

        [DllImport("user32.dll")]

        private static extern int SendMessage(IntPtr hWnd, int wMsg, int wParam, int lParam);
        

        private void treeView1_DragOver(object sender, DragEventArgs e)
        {
            // Set a constant to define the autoscroll region
            const Single scrollRegion = 20;

            // See where the cursor is
            Point pt = treeView1.PointToClient(Cursor.Position);

            // See if we need to scroll up or down
            if ((pt.Y + scrollRegion) > treeView1.Height)
            {
                // Call the API to scroll down
                SendMessage(treeView1.Handle, (int)277, (int)1, 0);
            }
            else if (pt.Y < (treeView1.Top + scrollRegion))
            {
                // Call thje API to scroll up
                SendMessage(treeView1.Handle, (int)277, (int)0, 0);
            }
        }