Heterogeneous Items

Hi,

I am trying to create a MultiColumnTreeView to represent the following data: A family object which has two properties, Name and the number of members belonging to the family (Count) as well as collection of Members of the family which has the following three properties (Name, DOB, Gender). I am trying to render the information as follows,

Name                             DOB                    Gender
+Robinson (2)

       Jane Robinson        01/01/08                 F

       John Robinson        01/01/08                 M
-Crusoe (3)

I am receiving binding exceptions due to the Family object not having properties for DOB and Gender as well as I am unable to display the number of members in the family. Any example or help as to how to go about remedying this problem would be greatly appreciated.

thanks - glenn.

The way around this is to use CellTemplateSelector instead of CellTemplate or DisplayMemberBinding.  In your CellTemplateSelector, examine the type of the item and hand off to a suitable DataTemplate or an empty template appropriately.  Here's an example from a sample we're working on at the moment.

In this example, we have a hierarchy of Countries, Companies and People, and our columns are Name, Contact Info and Notes.  Unfortunately for us, the Person class doesn't have a Name property, only FirstName and LastName properties; Companies have a set of address properties for their contact info, People have an EmailAddress property and Countries don't have contact info at all; and finally Countries don't have Notes.

The first thing we do is create a generic selector that just looks at the type of the item:

  public class EntityTypeTemplateSelector : DataTemplateSelector
  {
    public DataTemplate DefaultTemplate { get; set; }
    public DataTemplate PersonTemplate { get; set; }
    public DataTemplate CompanyTemplate { get; set; }
    public DataTemplate CountryTemplate { get; set; }

    public override DataTemplate SelectTemplate(object item, DependencyObject container)
    {
      // TODO: punting on extensibility for now
      if (item is Model.Person)
      {
        return PersonTemplate ?? DefaultTemplate;
      }
      if (item is Model.Company)
      {
        return CompanyTemplate ?? DefaultTemplate;
      }
      if (item is Model.Country)
      {
        return CountryTemplate ?? DefaultTemplate;
      }

      return DefaultTemplate;
    }
  }

As the comment indicates, I've ducked making this fully generic: it's tied specifically to my hierarchy.  This could be fixed by having a set of type-template pairs instead of the specific templates, but this makes the selector declarations in XAML a bit more verbose.  Let me know if you want to see that approach anyway.

Now we can start creating and selecting templates in our XAML.  I'll show the Name and Notes because that should be enough to give you the idea.

    <DataTemplate x:Key="EmptyTemplate">
      <TextBlock />
    </DataTemplate>

    <DataTemplate x:Key="DefaultNameTemplate">
      <TextBlock Text="{Binding Name}" Foreground="Black" />
    </DataTemplate>

    <DataTemplate x:Key="PersonNameTemplate">
      <StackPanel Orientation="Horizontal" TextBlock.Foreground="Black">
        <TextBlock Text="{Binding FirstName}" />
        <TextBlock Text=" " />
        <TextBlock Text="{Binding LastName}" />
      </StackPanel>
    </DataTemplate>

    <DataTemplate x:Key="DefaultNotesTemplate">
      <TextBlock Text="{Binding Notes}" Foreground="Black" />
    </DataTemplate>

    <local:EntityTypeTemplateSelector x:Key="NameSelector"
                                      DefaultTemplate="{StaticResource DefaultNameTemplate}"
                                      PersonTemplate="{StaticResource PersonNameTemplate}"
                                      />

    <local:EntityTypeTemplateSelector x:Key="NotesSelector"
                                      CountryTemplate="{StaticResource EmptyTemplate}"
                                      DefaultTemplate="{StaticResource DefaultNotesTemplate}"
                                      />

Finally we tell the MulticolumnTreeView to use these selectors:

      <ms:MulticolumnTreeView.Columns>
        <GridViewColumn Header="Name" CellTemplateSelector="{StaticResource NameSelector}" Width="150" />
        <GridViewColumn Header="Contact Info" CellTemplateSelector="{StaticResource ContactInfoSelector}" Width="180" />
        <GridViewColumn Header="Notes" CellTemplateSelector="{StaticResource NotesSelector}" Width="150" />
      </ms:MulticolumnTreeView.Columns>

This should address both your databinding errors (via the use of EmptyTemplate) and showing the number of members in the family (have your FamilyNameTemplate have text blocks bound to Name and Count, as we do above for Person FirstName and LastName).

Hope this makes sense -- let me know if you need any more info on this.