Question

I have this context menu resource:

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <ContextMenu x:Key="FooContextMenu">
        <ContextMenu.CommandBindings>
            <CommandBinding Command="Help" Executed="{Binding ElementName=MainTabs, Path=HelpExecuted}" />
        </ContextMenu.CommandBindings>

        <MenuItem Command="Help">
            <MenuItem.Icon>
                <Image Source="../Resources/Icons/Help.png" Stretch="None" />
            </MenuItem.Icon>
        </MenuItem>
    </ContextMenu>
</ResourceDictionary>

I want to re-use it in two places. Firstly I’m trying to put it in a DataGrid:

<DataGrid ContextMenu="{DynamicResource FooContextMenu}">...

The ContextMenu itself works fine, but with the Executed="..." I have right now breaks the application and throws:

A first chance exception of type ‘System.InvalidCastException’
occurred in PresentationFramework.dll

Additional information: Unable to cast object of type
‘System.Reflection.RuntimeEventInfo’ to type
‘System.Reflection.MethodInfo’.

If I remove the entire Executed="..." definition, then the code works (and the command does nothing/grayed out). The exception is thrown as soon as I right click the grid/open the context menu.

The DataGrid is placed under a few elements, but eventually they all are below a TabControl (called MainTabs) which has ItemsSource set to a collection of FooViewModels, and in that FooViewModel I have a method HelpExecuted which I want to be called.

Let’s visualize:

• TabControl (ItemsSource=ObservableCollection<FooViewModel>, x:Name=MainTabs)
  • Grid
    • More UI
      • DataGrid (with context menu set)

Why am I getting this error and how can I make the context menu command to “target” the FooViewModel‘s HelpExecuted method?

Answer 1

Unfortunately you cannot bind Executed for a ContextMenu as it is an event. An additional problem is that the ContextMenu does not exist in the VisualTree the rest of your application exists. There are solutions for both of this problems.

First of all you can use the Tag property of the parent control of the ContextMenu to pass-through the DataContext of your application. Then you can use an DelegateCommand for your CommandBinding and there you go. Here’s a small sample showing View, ViewModel and the DelegateCommand implementation you would have to add to you project.

DelegateCommand.cs

public class DelegateCommand : ICommand
{
    private readonly Action<object> execute;
    private readonly Predicate<object> canExecute;

    public DelegateCommand(Action<object> execute)
        : this(execute, null)
    { }

    public DelegateCommand(Action<object> execute, Predicate<object> canExecute)
    {
        if (execute == null)
            throw new ArgumentNullException("execute");

        this.execute = execute;
        this.canExecute = canExecute;
    }

    #region ICommand Members

    [DebuggerStepThrough]
    public bool CanExecute(object parameter)
    {
        return canExecute == null ? true : canExecute(parameter);
    }

    public event EventHandler CanExecuteChanged
    {
        add { CommandManager.RequerySuggested += value; }
        remove { CommandManager.RequerySuggested -= value; }
    }

    public void Execute(object parameter)
    {
        execute(parameter);
    }

    #endregion
}

MainWindowView.xaml

<Window x:Class="Application.MainWindowView"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindowView" Height="300" Width="300"
        x:Name="MainWindow">
    <Window.Resources>
        <ResourceDictionary>
            <ContextMenu x:Key="FooContextMenu">
                <MenuItem Header="Help" Command="{Binding PlacementTarget.Tag.HelpExecuted, RelativeSource={RelativeSource AncestorType=ContextMenu}}" />
            </ContextMenu>
        </ResourceDictionary>
    </Window.Resources>
    <Grid>
        <TabControl ItemsSource="{Binding FooViewModels}" x:Name="MainTabs">
            <TabControl.ContentTemplate>
                <DataTemplate>
                    <DataGrid ContextMenu="{DynamicResource FooContextMenu}" Tag="{Binding}" />
                </DataTemplate>
            </TabControl.ContentTemplate>
        </TabControl>
    </Grid>
</Window>

MainWindowView.xaml.cs

public partial class MainWindowView : Window
{
    public MainWindowView()
    {
        InitializeComponent();
        DataContext = new MainWindowViewModel();
    }
}

MainWindowViewModel.cs

public class MainWindowViewModel
{
    public ObservableCollection<FooViewModel> FooViewModels { get; set; }

    public MainWindowViewModel()
    {
        FooViewModels = new ObservableCollection<FooViewModel>();
    }
}

FooViewModel.cs

public class FooViewModel
{
    public ICommand HelpExecuted { get; set; }

    public FooViewModel()
    {
        HelpExecuted = new DelegateCommand(ShowHelp);
    }

    private void ShowHelp(object obj)
    {
        // Yay!
    }
}