Question

I have a problem with my data binding, probably a small one. I have tried to implement the MVVM pattern in my application. Therefore I have a model containig my data. This data gets updated over the network in a periodic way. On top of this model I have a view model to bind to. In this view model I have a ObservableCollection, which I want to bind. The problem I have is, that my view model needs to be a global resource. Thats why I use the following attempt in my NavigationWindow :

<NavigationWindow.DataContext>
    <localvm:DataViewModel/>
</NavigationWindow.DataContext>

The reason I have to use it as a global variable is, that this view model starts my network business logic. Thats probably a bad way but I could’nt find a solution to this. Anyway
to diplay my data I have a Frame in my NavigationWindow defined like this:

<Frame Source="/Views/Pages/Page1_SystemOverview.xaml" VerticalAlignment="Stretch></Frame>

in this loaded page I have a ItemsControl to view the Collection:

<Page>
<Grid>
    <Grid.Resources>
        <localcnv:DebugHelperConverter x:Key="debugCNV"/>
    </Grid.Resources>
    <StackPanel>
        <ItemsControl ItemsSource="{Binding Source=ListOfQuerys}">
            <ItemsControl.ItemsPanel>
                <ItemsPanelTemplate>
                    <StackPanel Orientation="Vertical"></StackPanel>
                </ItemsPanelTemplate>
            </ItemsControl.ItemsPanel>

        <ItemsControl.ItemTemplate>
            <DataTemplate>
                <StackPanel>
                        <Button Content="{Binding RelativeSource={RelativeSource Mode=TemplatedParent}, Path=(ItemsControl.AlternationIndex)}"/>
                </StackPanel>
            </DataTemplate>
        </ItemsControl.ItemTemplate>
    </ItemsControl>
    </StackPanel>   
</Grid>

So here is my problem: When I start my program The ItemsControl contains 18 elements with no content at all. It should contain only two elements, the output window shows me no binding errors, but when I change the ItemsSource to

"{Binding Path=ListOfQuerys}"

it says:

System.Windows.Data Information: 41 : BindingExpression path error: 'ListOfQuerys' property not found for 'object' because data item is null.  This could happen because the data provider has not produced any data yet. BindingExpression:Path=ListOfQuerys; DataItem=null; target element is 'ItemsControl' (Name=''); target property is 'ItemsSource' (type 'IEnumerable')

What am I doing wrong? I thought, when I set the DataContext in a parent class, I can use it down the tree but somehow it won’t work, any Ideas?

Thanks!

Answer 1

The content of the Frame element is not part of visual / logical tree and hence will not persist the data context. You will have to set it explicitly.

    <Frame Source="Page1.xaml" Navigated="Frame_Navigated"/>

    private void Frame_Navigated(object sender, System.Windows.Navigation.NavigationEventArgs e)
    {
        ((FrameworkElement) e.Content).DataContext = this.DataContext;
    }

Hope this helps…