The DataContext is one of the most important parts of the WPF data binding system, especially in MVVM applications. Being built into the Binding type as the common source for bindings in a specific scope reduces plumbing code needed and makes XAML more concise. Unfortunately, for any given element there is only one DataContext available to bind against. Most of the time this isn’t a problem, but complex data hierarchies often lead to situations where an element needs to bind to the DataContext of some parent element in addition to its own local DataContext.

I most often run into this with ItemsControls where something inside the ItemTemplate needs to bind to the ViewModel containing the entire collection:

<ItemsControl ItemsSource="{Binding Items}">
    <ItemsControl.ItemTemplate>
        <DataTemplate>
            <DockPanel>
                <TextBlock Text="{Binding Name}"/>
                <Button Content="Remove" Command="<Remove item command on parent VM>" CommandParameter="{Binding}"/>
            </DockPanel>
        </DataTemplate>
    </ItemsControl.ItemTemplate>
</ItemsControl>

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