CSharp Code:
  1. private void ListBox1_MouseDoubleClick(object sender, MouseEventArgs e)
  2. {
  3.     if(this.ListBox1.IndexFromPoint(e.Location) != ListBox.NoMatches)
  4.     {
  5.         MessageBox.Show(this.ListBox1.GetItemText(this.ListBox1.SelectedItem));
  6.     }
  7. }