private void ClearSelectionBtn_Click(object sender, EventArgs e)
 {
     for (int i = 0; i < ExtensionChckdListBox.Items.Count; i++)
     {
         ExtensionChckdListBox.SetItemChecked(i, false);
     }
 }
        private void ReturnSelectionBtn_Click(object sender, EventArgs e)
        {
            if (ExtensionChckdListBox.CheckedItems.Count == 0)
            {
                return;
            }

            EXTNAMES            = new string[ExtensionChckdListBox.CheckedItems.Count];
            EXTINDEXES_ONEBASED = new int[ExtensionChckdListBox.CheckedItems.Count];

            for (int i = 0; i < ExtensionChckdListBox.Items.Count; i++)
            {
                if (ExtensionChckdListBox.GetItemChecked(i))
                {
                    EXTNAMES[i]            = ExtensionChckdListBox.Items[i].ToString();
                    EXTINDEXES_ONEBASED[i] = i + 1;
                }
            }

            this.DialogResult = DialogResult.OK;
            this.Close();
        }