Why does this simple databinding scenario not work? (ComboBox related)

Question

Hi all,

I've been scratching my head on this one for a while now and am stumped at the moment.

The problem scenario is easier to explain as code so hopefully it speaks for itself. First of all, I have a silverlight application with the following in the XAML...

<UserControl x:Class="SilverlightApplication2.Page"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
Width="400" Height="300">

<UserControl.Resources>
    <DataTemplate x:Key="icTemplate">
        <ComboBox ItemsSource="{Binding StringsChild}" SelectedItem="{Binding SelectedItem}"/>
    </DataTemplate>
</UserControl.Resources>

<Grid x:Name="LayoutRoot" Background="White">
    <Grid.RowDefinitions>
        <RowDefinition/>
        <RowDefinition/>
    </Grid.RowDefinitions>
    <ItemsControl x:Name="ic" ItemTemplate="{StaticResource icTemplate}">
        <ItemsControl.ItemsPanel>
            <ItemsPanelTemplate>
                <StackPanel Orientation="Vertical"/>
            </ItemsPanelTemplate>
        </ItemsControl.ItemsPanel>
    </ItemsControl>

    <Button Click="Save" Grid.Row="1" Content="GO"/>
</Grid>

My code-behind looks like this...(all written in a single class file so that it's easy for you to copy it into your own project and compile)

namespace SilverlightApplication2
{
    public partial class Page : UserControl
    {
        public ObservableCollection<SomeClass> StringsParent { get; set; } 

        public Page()
        {   
            InitializeComponent();
            StringsParent = new ObservableCollection<SomeClass>();
            ic.ItemsSource = StringsParent;
        }

        private void Save(object sender, RoutedEventArgs e)
        {
            SomeClass c = new SomeClass();
            c.StringsChild.Add("First");
            c.StringsChild.Add("Second");
            c.StringsChild.SetSelectedItem("Second");
            StringsParent.Add(c);
        }
    }

    public class SomeClass
    {
        public SelectableObservablecollection<string> StringsChild { get; set; }

        public SomeClass()
        {
            StringsChild = new SelectableObservablecollection<string>();
        }    
    }

    public class SelectableObservablecollection<T> : ObservableCollection<T>
    {
        public SelectableObservablecollection()
            : base()
        {

        }

        public void SetSelectedItem<Q>(Q selectedItem)
        {
            foreach (T item in this)
            {
                if (item.Equals(selectedItem))
                {
                    SelectedItem = item;
                    return;
                }
            }
        }

        private T _selectedItem;
        public T SelectedItem
        {
            get
            {
                return _selectedItem;
            }
            set
            {
                _selectedItem = value;
                OnPropertyChanged(new PropertyChangedEventArgs("SelectedItem"));
            }
        }
    }
}

So let me explain... I set out to write a generic way of creating an ObservableCollection that has a SelectedItem property on it so that when I bind the collection to a ComboBox for example, I can Bind the ComboBox's SelectedItem property to it.

However, for some reason, it does not seem to work when the ComboBox is effectively nested via an ItemTemplate. I effectively have a list of lists, a scenario which is simple enough that I'm lost as to what's wrong.

When you run the code you'll see that the templated ComboBox does pick up the correct items, but it's never set to a SelectedItem despite the binding.

I know it's rather long winded, but...any ideas?

Thanks alot

Answer 1

The debugger output actually gives you a hint to the problem:

System.Windows.Data Error: BindingExpression path error: 'SelectedItem' property not found on 'ExpressionElements.SomeClass' 'ExpressionElements.SomeClass' (HashCode=49044892). BindingExpression: Path='SelectedItem' DataItem='ExpressionElements.SomeClass' (HashCode=49044892); target element is 'System.Windows.Controls.ComboBox' (Name=''); target property is 'SelectedItem' (type 'System.Object')..

Because the Data context for the template is an instance of the SomeClass class, all you have to do is change the SelectedItem binding from SelectedItem to StringsChild.SelectedItem:

<DataTemplate x:Key="icTemplate">
    <ComboBox ItemsSource="{Binding StringsChild}" 
     SelectedItem="{Binding StringsChild.SelectedItem}"/>
</DataTemplate>