WPF ContextMenu woes: How do I set the DataContext of the ContextMenu?

Question

I am having some trouble figuring out how to set the correct DataContext on a ContextMenu.

I have a collection of view models who are the source of an ItemsControl. Each view model has a collection of items which are also the source of another ItemsControl. Each item is used to draw image which has a ContextMenu. The MenuItems in that ContextMenu need to bind to a command on the view model, but the PlacementTarget of the ContextMenu is pointing to the individual item.

My Xaml looks something like this:

<ItemsControl ItemsSource="{Binding Markers"}>
    <ItemsControl.ItemTemplate>
        <DataTemplate>
            <ItemsControl ItemsSource="{Binding Items}">
                <ItemsControl.ItemTemplate>
                    <DataTemplate>
                        <Image>
                            <Image.ContextMenu>
                                <ContextMenu>
                                     <MenuItem Header="Edit" Command="{Binding EditCommand}" />
                                </ContextMenu>
                            </Image.ContextMenu>
                        </Image>
                    </DataTemplate>
                </ItemsControl.ItemTemplate>
            </ItemsControl>
        </DataTemplate>
    </ItemsControl.ItemTemplate>
</ItemsControl>

How can I set the DataContext of the ContextMenu to the item's corresponding parent view model?

Answer 1

The ContextMenu is outside of the visual tree. Below is the xaml that should get you the datacontext:

<ItemsControl ItemsSource="{Binding Markers}" Tag="{Binding ElementName=outerControl, Path=DataContext}">
   ...
   <ContextMenu DataContext="{Binding Path=PlacementTarget.Tag, RelativeSource={RelativeSource Self}}">
      <MenuItem Header="Edit"
                Command="{Binding EditCommand}" />
   </ContextMenu>
   ...
</ItemsControl>

This post explains how this works.

Answer 2

You can use a markupextension:

using System;
using System.Windows.Controls;
using System.Windows.Markup;
using System.Xaml;

[MarkupExtensionReturnType(typeof(ContentControl))]
public class RootObject : MarkupExtension
{
    public override object ProvideValue(IServiceProvider serviceProvider)
    {
        var rootObjectProvider = (IRootObjectProvider)serviceProvider.GetService(typeof(IRootObjectProvider));
        return rootObjectProvider?.RootObject;
    }
}

It lets you do:

<ItemsControl ItemsSource="{Binding Markers}">
   ...
   <ContextMenu DataContext="{Binding DataContext, Source={local:RootObject}}">
      <MenuItem Header="Edit"
                Command="{Binding EditCommand}" />
   </ContextMenu>
   ...
</ItemsControl>

Answer 3

I don't like use Tag. I prefer attached property.

You need add attached property:

public static readonly DependencyProperty DataContextExProperty =
   DependencyProperty.RegisterAttached("DataContextEx",
                                       typeof(Object), 
                                       typeof(DependencyObjectAttached));

public static Object GetDataContextEx(DependencyObject element)
{
    return element.GetValue(DataContextExProperty);
}

public static void SetDataContextEx(DependencyObject element, Object value)
{
    element.SetValue(DataContextExProperty, value);
}

In XAML:

<Button attached:DependencyObjectAttached.DataContextEx="{Binding ElementName=MyDataContextElement, Path=DataContext}">
    <Button.ContextMenu>
        <ContextMenu DataContext="{Binding RelativeSource={RelativeSource Self}, Path=PlacementTarget.(attached:DependencyObjectAttached.DataContextEx)}">
        </ContextMenu>
    </Button.ContextMenu>
</Button>

Answer 4

This code allows you to use both the global and local DataContext (of entire userControl and of current TreeViewItem):

<TreeView Grid.Row="0" ItemsSource="{Binding Path=SelectableEnterprises}">
<TreeView.ItemTemplate>
    <HierarchicalDataTemplate ItemsSource="{Binding Children}">
        <Grid Height="20" Tag="{Binding DataContext, ElementName=userControl}">
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="Auto"/>
                <ColumnDefinition Width="25"/>
                <ColumnDefinition/>
            </Grid.ColumnDefinitions>
            <Grid.ContextMenu>
                <ContextMenu DataContext="{Binding PlacementTarget, RelativeSource={RelativeSource Self}}" >
                    <ContextMenu.Visibility>
                        <MultiBinding Converter="{converters:SurveyReportVisibilityConverter}">
                            <Binding Path="DataContext"/> <!--local DataContext-->
                            <Binding Path="Tag.SelectedSurvey"/> <!--global DataContext-->
                        </MultiBinding>
                    </ContextMenu.Visibility>

                    <MenuItem Header="Show HTML-report" Command ="{Binding Tag.OpenHTMLReportCommand}" 
                              CommandParameter="{Binding DataContext}" />
                </ContextMenu>
            </Grid.ContextMenu>