First off, I've returned from a great 7 weeks holiday in New Zealand. I will not bore you with 1500 pictures, although I am perfectly capable of doing just that. On top of that, 3 hours of film was shot. Yikes. It was just that incredible!

For picking up the blogging slack, i'll share with you guys a mechanism for extending objects with attached properties. Although the technique is well documented, I didn't find any resources online with an example of a checked listbox. I did find a post by Josh Smith about it, but he created the checked listbox using the Checked and Unchecked events, which I don't feel is very 'WPF' like.. Not when you are using WPF! So, let's first look at the problem at hand, and then get on to the solution, which is so simple, it feels like an anti-climax.

We have an ObservableCollection<T> with domainobjects, or in our case simple Strings. We wish to show these in a listbox, and give the user the abiliby to select multiple items using checkboxes. We also provide a button, which will just select all of them. Obviously, we want to be able to get to the checked items.
In the past, I would probably have used checked and unchecked events to keep track of the checked items, or I would have extended my domainobject with a 'IsSelected' property and bind to it. The latter would have made me extremely unhappy, because it would mean my userinterface was invading my domainobject.

An attached property, basically, is a property that belongs to another object, but can be set on any dependency object you choose. So, I will introduce a boolean attached property (for instance: IsGeselecteerd, dutch for IsSelected). Then I create the datatemplate for my objects, with a checkbox in it, and a binding to the attachedproperty:

  <Window.Resources>
    <DataTemplate x:Key="lbitems">
      
        <StackPanel Orientation="Horizontal">
          <CheckBox Name="checkbox" l:Window1.Geselecteerd="{Binding Path=IsChecked, RelativeSource={RelativeSource Self}, Mode=TwoWay }" />
          <Label Content="{Binding}" />
        </StackPanel>
      
    </DataTemplate>
  </Window.Resources>
    <StackPanel>
      <ListBox Name="lv" ItemsSource="{Binding Path=Lijst}" ItemTemplate="{StaticResource lbitems}">
        </ListBox>

      <Button Click="SelectAll">Select all</Button>
      <Button Click="LeesUit">show selected items in debug.output</Button>
    </StackPanel>
</Window>

The binding of interest is: l:Window1.Geselecteerd="{Binding Path=IsChecked, RelativeSource={RelativeSource Self}, Mode=TwoWay }", on the checkbox.
It means that the IsChecked property of itself (the checkbox) is bound to the property Geselecteerd, which lives on the Window1 class.

That's basically all there is to it. Setting the checkboxes from procedural code is somewhat harder then I would like it to be, but only because I couldn't find an easy way to get to the checkbox from code:

            foreach(object dataitem in lv.Items)
            {
                // get the visual container, belonging to the dataitem
                ListBoxItem lbitem = (ListBoxItem)lv.ItemContainerGenerator.ContainerFromItem(dataitem);

                // get to the checkbox
                CheckBox c = (CheckBox) GeefChildHelper(lbitem, "checkbox");


                // set the attached property 
                SetGeselecteerd(c, true);
            }

Point of interest is the setting of the attached property, which will tell the binding of the checkbox to do the appropriate thing and the Helper method, to get to the checkbox:

        private object GeefChildHelper(ListBoxItem lbitem, string naam)
        {
            Border border = VisualTreeHelper.GetChild(lbitem, 0) as Border;
            ContentPresenter contentPresenter = VisualTreeHelper.GetChild(border, 0) as ContentPresenter;
            return lv.ItemTemplate.FindName(naam, contentPresenter);

        }

Here you see the need to first find a contentpresenter, before the FindName method will work..... That's because apparently that's the thing the template is bound to.

Reading the values is just as easy:

            foreach(string dataitem in Lijst)
            {
                ListBoxItem lbitem = (ListBoxItem)lv.ItemContainerGenerator.ContainerFromItem(dataitem);
                CheckBox c = (CheckBox)GeefChildHelper(lbitem, "checkbox");

                Debug.WriteLine(String.Format("item: {0} is {1}", dataitem, GetGeselecteerd(c).ToString()));
            }

I hope this sample helps someone!