Question

Solved it myself. It was the way I initialised the Settings collection. Specifying a default when registering it as a DependencyProperty causes all of the Settings to refer to the same collection object. Adding a constructor to Category and explicitly initialising Settings resolves the issue.

---

A class Category specifies a name and a collection of Settings objects.

using System.Collections.ObjectModel;
using System.Windows;

namespace CasEdit
{
  public class Categories : ObservableCollection<Category> { }

  public class Category : DependencyObject
  {
    public string Caption
    {
      get { return (string)GetValue(CategoryProperty); }
      set { SetValue(CategoryProperty, value); }
    }
    public static readonly DependencyProperty CategoryProperty =
        DependencyProperty.Register("Caption", typeof(string), typeof(Category), 
        new UIPropertyMetadata("Category name not set"));
    public Settings Settings
    {
      get { return (Settings)GetValue(SettingsProperty); }
    }
    public static readonly DependencyProperty SettingsProperty =
        DependencyProperty.Register("Settings", typeof(Settings), typeof(Category), 
        new UIPropertyMetadata(new Settings()));
  }
}

The following XAML defines templates, UI and some test data.

<Window x:Class="CasEdit.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:CasEdit="clr-namespace:CasEdit"
        Title="MainWindow" Height="350" Width="525" >
  <Window.Resources>
    <HierarchicalDataTemplate DataType="{x:Type CasEdit:Category}" ItemsSource="{Binding Settings}">
      <StackPanel Orientation="Horizontal">
        <TextBlock Text="{Binding Caption}" />
        <Button Content="Gratuitous button" Margin="3" Click="Button_Click"/>
      </StackPanel>
    </HierarchicalDataTemplate>
    <DataTemplate DataType="{x:Type CasEdit:Setting}" >
      <StackPanel Orientation="Horizontal">
        <TextBlock Text="{Binding Caption}" Margin="3" />
        <TextBlock Text="{Binding Template}" Margin="3" />
        <TextBox Text="{Binding Editor}" Margin="3" />
        <TextBlock Text="{Binding CasDevice}" Margin="3" />
      </StackPanel>
    </DataTemplate>
    <CasEdit:Categories x:Key="cats">
      <CasEdit:Category Caption="1st category">
        <CasEdit:Category.Settings>
          <CasEdit:Setting Caption="Setting 1-1" />
          <CasEdit:Setting Caption="Setting 1-2" />
        </CasEdit:Category.Settings>
      </CasEdit:Category>
      <CasEdit:Category Caption="2nd category" >
        <CasEdit:Category.Settings>
          <CasEdit:Setting Caption="Setting 2-1" />
        </CasEdit:Category.Settings>        
      </CasEdit:Category>
      <CasEdit:Category Caption="3rd category" >
        <CasEdit:Category.Settings>
          <CasEdit:Setting Caption="Setting 3-1" />
        </CasEdit:Category.Settings>        
      </CasEdit:Category>
    </CasEdit:Categories>
  </Window.Resources>
  <Grid>
    <TreeView x:Name="tree" ItemsSource="{Binding Source={StaticResource cats}}" />
  </Grid>
</Window>

You would expect a tree like this

• 1st category
  • Setting 1-1
  • Setting 1-2
• 2nd Category
  • Setting 2-1
• 3rd category
  • Setting 3-1

but what I get is this

which is very confusing. Where have I gone astray, that each category shows all of the settings?

Answer 1

The last parameter here is telling making it so that every instance of Category has it’s Settings property initialized to point to the same object:

public static readonly DependencyProperty SettingsProperty =   
        DependencyProperty.Register("Settings", typeof(Settings), typeof(Category),    
        new UIPropertyMetadata(new Settings()));   

Instead, do this:

    public static readonly DependencyProperty SettingsProperty =
        DependencyProperty.Register("Settings", typeof(Settings), typeof(Category),
        new UIPropertyMetadata(null));
    public Category()
    {
        Settings = new Settings();
    }