ListBox上の座標により適切なアイテムを選択する方法

private void Form1_Load(object sender, System.EventArgs e)
{
    for(int i = 0;i<10;i++)
        listBox1.Items.Add(string.Format("アイテム={0}",i));
    for(int i = 0;i<10;i++)
        listBox2.Items.Add(string.Format("アイテム={0}",i));
    listBox2.AllowDrop = true;
}

private void listBox1_MouseDown(object sender, System.Windows.Forms.MouseEventArgs e)
{
    if ((e.Button == MouseButtons.Left) && (listBox1.SelectedItem != null))
    {
        int idx = listBox1.IndexFromPoint(listBox1.PointToClient(new Point(e.X,e.Y)));
        string str = (string)listBox1.Items[listBox1.SelectedIndex];
        listBox1.DoDragDrop(str,DragDropEffects.Copy);
    }
}

private void listBox2_DragOver(object sender, System.Windows.Forms.DragEventArgs e)
{
    int idx = listBox2.IndexFromPoint(listBox2.PointToClient(new Point(e.X,e.Y)));
    string source = (string)e.Data.GetData("System.String");
    string target = (string)listBox2.Items[idx];
    e.Effect = (string.Compare(source,target) == 0)?DragDropEffects.Copy:DragDropEffects.None;
}