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<ComboBox Name="help3Catalogs"
  ItemsSource="{Binding}" DisplayMemberPath="ShortName" SelectedValuePath="ShortName"
  SelectionChanged="Help3CatalogsSelectionChanged" />                    

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void Help3CatalogsSelectionChanged(object sender, SelectionChangedEventArgs e)
{
  string item = (string)help3Catalogs.SelectedValue;
  if (!string.IsNullOrEmpty(item)) {
    Help3Service.ActiveCatalogId = item;
  }
}                    

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<ComboBox.ItemTemplate>
    <DataTemplate>
        <TextBlock Text="{Binding ...}" />
    </DataTemplate>
</ComboBox.ItemTemplate>                    

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<ComboBox Name="searchCB" Margin="5,1,5,5" Padding="1" IsEnabled="False" IsEditable="True" PreviewKeyUp="SearchCBPreviewKeyUp" />                    

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ObservableCollection<string> searchTerms = new ObservableCollection<string>();

void SearchCBPreviewKeyUp(object sender, KeyEventArgs e)
{
  doSearch.IsEnabled = (Help3Environment.IsHelp3ProtocolRegistered && !string.IsNullOrEmpty(searchCB.Text));

  if (e.Key == Key.Return || e.Key == Key.Enter)
    DoSearchClicked(null, null);
}

void DoSearchClicked(object sender, RoutedEventArgs e)
{
  string term = searchCB.Text;
  if (string.IsNullOrEmpty(term)) {
    throw new ArgumentNullException("term");
  }
  if (searchTerms.IndexOf(term) < 0) searchTerms.Add(term);
  DisplayHelp.Search(term);
}