Question

I have created a user control with collection property:

    public static readonly DependencyProperty
    MyListProperty = DependencyProperty.Register(
        "MyList",
        typeof(ObservableCollection<Test>),
        typeof(UserControl1),
        new FrameworkPropertyMetadata(new ObservableCollection<Test>())
        );

    public ObservableCollection<Test> MyList
    {
        get { return (ObservableCollection<Test>)base.GetValue(MyListProperty); }
        set { base.SetValue(MyListProperty, value); }
    }

    public static readonly DependencyProperty
    BProperty = DependencyProperty.Register(
        "B",
        typeof(string),
        typeof(UserControl1),
        new FrameworkPropertyMetadata(null)
    );

    public string B
    {
        get { return (string)base.GetValue(BProperty); }
        set { base.SetValue(BProperty, value); }
    }

The Test class is:

public class Test : DependencyObject
{
    public static readonly DependencyProperty
    AProperty = DependencyProperty.Register(
        "A",
        typeof(string),
        typeof(Test),
        new FrameworkPropertyMetadata(null)
    );

    public string A 
    {
        get { return (string)base.GetValue(AProperty); }
        set { base.SetValue(AProperty, value); }
    }
}

Then, i’m trying to use my control for binding:

    <TextBox x:Name="tb1" Text="def"/>
    <my:UserControl1 x:Name="uc1" B="{Binding ElementName=tb1, Path=Text}">
        <my:UserControl1.MyList>
            <my:Test A="{Binding ElementName=tb1, Path=Text}"></my:Test>
            <my:Test A="100"></my:Test>
        </my:UserControl1.MyList>
    </my:UserControl1>

The first binding (with B property of User Control) works correctly. The problem is with second binding (with A property of Test which is MyList element). When debugging i have two items in MyList but the A property of the first one is null. Please tell me what I am missing here?

Answer 1

The problem here is, that the Binding to ElementName=tb1 can not be resolved, even it will never be evaluated. A Binding to an ElementName is resolved for DependencyObjects which are in the visual or logical Tree of a WPF Application. Adding items to your ObservableCollection (MyList) only means adding the items to the Collection, but not into the Visual Tree.

Edit:
Here is the approach discussed in the comments:

In your Window/Page:

<Window.Resources>
    <!-- Declare the ViewModel as Resource -->
    <my:ViewModel x:Key="viewModel">
        <my:ViewModel.MyList>
            <my:Test A="Hello sweet" />
            <my:Test A="ViewModel" />
        </my:ViewModel.MyList>
    </my:ViewModel>
</Window.Resources>

<!-- Assign Ressource as DataContext -->
<StackPanel DataContext="{StaticResource viewModel}">

    <TextBox x:Name="tb1" Text="def"/>

    <!-- Reference your list within the ViewModel -->
    <ListBox ItemsSource="{Binding Path=MyList}">
        <ListBox.ItemTemplate>
            <DataTemplate>
                <!-- Bind your property  -->
                <TextBlock Text="{Binding Path=A}" />
            </DataTemplate>
        </ListBox.ItemTemplate>
    </ListBox>
</StackPanel>

And the implementation of ViewModel:

public class ViewModel
{
    public ViewModel()
    {
        this.MyList = new ObservableCollection<Test>();
    }

    public ObservableCollection<Test> MyList { get; set; }
}

Of course, class Test no longer needs to implement a DependencyObject. Simple get/set Properties are okay.