Command Binding in hierarchical datatemplate

Question

I have Menu in my app. I'm visualizing it using hierarchical data template:

    <MenuItem Header="Main menu" ItemsSource="{Binding ApplicationMenu}" >
        <MenuItem.ItemTemplate>                    
            <HierarchicalDataTemplate DataType="{x:Type tm:RMenuItem}" 
                                      ItemsSource="{Binding Path=ChildrenItems}">                        
                <MenuItem Header="{Binding Name}" Command="{Binding RunOperationCommand}" />
            </HierarchicalDataTemplate>
        </MenuItem.ItemTemplate>
    </MenuItem>

menu looks like as it should, but Command for each menu item is not fired! Even more - it is not bounded, which could be seen in debugger: get accessor of ICommand Property has been never executed. Why it happens so?

Doing as usual works perfect:

<Menu>
    <MenuItem Header="SomeHeader" Command="{Binding RunOperationCommand}"/>
<Menu>

Answer 1

The difference between the first and the second example in your question is that in the second code snippet you are binding MenuItem.Command to the parent's data context, which has the RunOperationCommand defined. Whereas in the first example with the HierarchicalDataTemplate you are binding to the "local" DataContext, which is a menu item. It doesn't have the appropriate property, so the binding fails.

You have several options:

• one is to extend your menu items with the command property, like you did in your answer already;
• bind to the relative source up in the visual tree, which has the data context with the command, e.g. assuming that the command is in your window's DataContext:

---

    <MenuItem Header="Main menu" ItemsSource="{Binding ApplicationMenu}" >
        <MenuItem.ItemTemplate>                    
            <HierarchicalDataTemplate DataType="{x:Type tm:RMenuItem}" 
                                      ItemsSource="{Binding Path=ChildrenItems}">                        
                <MenuItem Header="{Binding Name}" 
                          Command="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Window}}, Path=DataContext.RunOperationCommand}" 
                />
            </HierarchicalDataTemplate>
        </MenuItem.ItemTemplate>
    </MenuItem>

---

• Make a StaticResource from your command, similar to this blog post approach

---

<Window.Resources>
     <coreView:CommandReference x:Key="RunOperationCommand"
                                Command="{Binding RunOperationCommand}" />
</Window.Resources>

    <MenuItem Header="Main menu" ItemsSource="{Binding ApplicationMenu}" >
        <MenuItem.ItemTemplate>                    
            <HierarchicalDataTemplate DataType="{x:Type tm:RMenuItem}" 
                                      ItemsSource="{Binding Path=ChildrenItems}">                        
                <MenuItem Header="{Binding Name}" 
                          Command="{StaticResource RunOperationCommand}" 
                />
            </HierarchicalDataTemplate>
        </MenuItem.ItemTemplate>
    </MenuItem>