mousedown cancels checkonclick

i've got a checkedlistbox with a mousedown event which starts a dragNdrop procedure. one can move entries up and down in the list. unfortunatly this event cancels the checkonclick property of the list. even if i write a mouseclick event for checking the boxes it doesn't work, the event just ins't triggered. what am i doing wrong?

private void lbx_MouseDown(object sender, System.Windows.Forms.MouseEventArgs e)
{
    if (((CheckedListBox)sender).Items.Count == 0)
        return;
 
    int index = ((CheckedListBox)sender).IndexFromPoint(e.X, e.Y);
 
    //set to last entry
    if (index == -1)
        return;
 
    Sp sp = (Sp)((CheckedListBox)sender).Items[index];
    DragDropEffects dde1 = DoDragDrop(sp, DragDropEffects.Move);
}
 
private void lbx_DragEnter(object sender, System.Windows.Forms.DragEventArgs e)
{
    e.Effect = DragDropEffects.Move;            
}
 
private void lbx_DragDrop(object sender, System.Windows.Forms.DragEventArgs e)
{
    Sp sp = (Sp) e.Data.GetData(typeof (Sp));
    int indexPos = ((CheckedListBox) sender).IndexFromPoint(((CheckedListBox) sender).PointToClient(new Point(e.X, e.Y)));
 
    ((CheckedListBox) sender).Items.Remove(sp);
 
    if (indexPos > -1)
        ((CheckedListBox) sender).Items.Insert(indexPos, sp);
    else
        ((CheckedListBox) sender).Items.Add(sp);
}
 
private void lbx_MouseClick(object sender, System.Windows.Forms.MouseEventArgs e)
{
    int index = ((CheckedListBox)sender).IndexFromPoint(e.X, e.Y);
    ((CheckedListBox)sender).SetItemChecked(index, !((CheckedListBox)sender).GetItemChecked(index));
}

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