void __fastcall TForm1::Button1Click(TObject *Sender)
{
  for( int i = 0; i < ListBox1->Items->Count; i++)
  {
    if(ListBox1->Selected[i])
    {
      ShowMessage("Selected item index:" + IntToStr(i));
      return;
    }
  }  
  ShowMessage("Not Selected any item");
}