XAML
  <ComboBox Name="cbBox" SelectionChanged="cbBox_SelectionChanged"
    MouseDoubleClick="cbBox_MouseDoubleClick"  />


  <ComboBox Name="cbBox" SelectionChanged="cbBox_SelectionChanged"
    MouseDoubleClick="cbBox_MouseDoubleClick"  >
    <ComboBoxItem>abc</ComboBoxItem>
    <ComboBoxItem>def</ComboBoxItem>
    <ComboBoxItem>ghi</ComboBoxItem>
  </ComboBox>

CS

  private initListeBox1() {
    cbBox.Items.Clear();
    cbBox.Items.Add("abc");
    cbBox.Items.Add("def");
    cbBox.Items.Add("ghi");
    cbBox.Items.Add("jkl");
    if (cbBox.Items.Count > 0) {
      cbBox.SelectedIndex = 0;
    }
  }

  private void cbBox_SelectionChanged(object sender, SelectionChangedEventArgs e)  {
    showFields();
  }

  private void showFields() {
     int i = cbBox.SelectedIndex;
     if ((!start) && (i >= 0)) {
         string table = (string)listBox1.SelectedItem;
     }
  }