WPF Context menu doesn't bind to right databound item

Question

Hi! I have a problem when binding a command in a context menu on a usercontrol that is on a tab page. The first time I use the menu (right-click on the tab) it works great, but if I switch tab the command will use the databound instance that was used the first time.

If I put a button that is bound to the command in the usercontrol it works as expected...

Can someone please tell me what I'm doing wrong??

This is a test project that exposes the problem:

App.xaml.cs:

public partial class App : Application
{
    protected override void OnStartup(StartupEventArgs e)
    {
        base.OnStartup(e);

        CompanyViewModel model = new CompanyViewModel();
        Window1 window = new Window1();
        window.DataContext = model;
        window.Show();
    }
}

Window1.xaml:

<Window x:Class="WpfApplication1.Window1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:vw="clr-namespace:WpfApplication1"
Title="Window1" Height="300" Width="300">

  <Window.Resources>
    <DataTemplate x:Key="HeaderTemplate">
        <StackPanel Orientation="Horizontal">
            <TextBlock Text="{Binding Path=Name}" />
        </StackPanel>
    </DataTemplate>
    <DataTemplate DataType="{x:Type vw:PersonViewModel}">
        <vw:UserControl1/>
    </DataTemplate>

</Window.Resources>
<Grid>
    <TabControl ItemsSource="{Binding Path=Persons}" 
                ItemTemplate="{StaticResource HeaderTemplate}"
                IsSynchronizedWithCurrentItem="True" />
</Grid>
</Window>

UserControl1.xaml:

<UserControl x:Class="WpfApplication1.UserControl1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    MinWidth="200">
    <UserControl.ContextMenu>
        <ContextMenu >
            <MenuItem Header="Change" Command="{Binding Path=ChangeCommand}"/>
        </ContextMenu>
    </UserControl.ContextMenu>
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="100" />
            <ColumnDefinition Width="*" />
        </Grid.ColumnDefinitions>
        <Label Grid.Column="0">The name:</Label>
        <TextBox Grid.Column="1" Text="{Binding Path=Name, UpdateSourceTrigger=PropertyChanged}" />
    </Grid>
</UserControl>

CompanyViewModel.cs:

public class CompanyViewModel
{
    public ObservableCollection<PersonViewModel> Persons { get; set; }
    public CompanyViewModel()
    {
        Persons = new ObservableCollection<PersonViewModel>();
        Persons.Add(new PersonViewModel(new Person { Name = "Kalle" }));
        Persons.Add(new PersonViewModel(new Person { Name = "Nisse" }));
        Persons.Add(new PersonViewModel(new Person { Name = "Jocke" }));
    }
}

PersonViewModel.cs:

public class PersonViewModel : INotifyPropertyChanged
{
    Person _person;
    TestCommand _testCommand;

    public PersonViewModel(Person person)
    {
        _person = person;
        _testCommand = new TestCommand(this);
    }
    public ICommand ChangeCommand 
    {
        get
        {
            return _testCommand;
        }
    }
    public string Name 
    {
        get
        {
            return _person.Name;
        }
        set
        {
            if (value == _person.Name)
                return;
            _person.Name = value;
            OnPropertyChanged("Name");
        }
    }
    void OnPropertyChanged(string propertyName)
    {
        PropertyChangedEventHandler handler = this.PropertyChanged;
        if (handler != null)
        {
            var e = new PropertyChangedEventArgs(propertyName);
            handler(this, e);
        }
    }
    public event PropertyChangedEventHandler PropertyChanged;
}

TestCommand.cs:

public class TestCommand : ICommand
{
    PersonViewModel _person;
    public event EventHandler CanExecuteChanged;

    public TestCommand(PersonViewModel person)
    {
        _person = person;
    }
    public bool CanExecute(object parameter)
    {
        return true;
    }
    public void Execute(object parameter)
    {
        _person.Name = "Changed by command";
    }
}

Person.cs:

public class Person
{
    public string Name { get; set; }
}

Answer 1

The key thing to remember here is context menus are not part of the visual tree.

Therefore they don't inherit the same source as the control they belong to for binding. The way to deal with this is to bind to the placement target of the ContextMenu itself.

<MenuItem Header="Change" Command="{Binding 
    Path=PlacementTarget.ChangeCommand, 
    RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ContextMenu}}}"
/>

Answer 2

The cleanest way I have found to bind commands to context menu items involves using a class called CommandReference. You can find it in the MVVM toolkit on Codeplex at WPF Futures.

The XAML might look like this:

<UserControl x:Class="View.MyView" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                xmlns:vm="clr-namespace:ViewModel;assembly=MyViewModel"
                xmlns:mvvm="clr-namespace:ViewModelHelper;assembly=ViewModelHelper"
           <UserControl.Resources>
                <mvvm:CommandReference x:Key="MyCustomCommandReference" Command="{Binding MyCustomCommand}" />

                <ContextMenu x:Key="ItemContextMenu">
                    <MenuItem Header="Plate">
                        <MenuItem Header="Inspect Now" Command="{StaticResource MyCustomCommandReference}"
                                CommandParameter="{Binding}">
                        </MenuItem>
                    </MenuItem>
               </ContextMenu>
    </UserControl.Resources>

MyCustomCommand is a RelayCommand on the ViewModel. In this example, the ViewModel was attached to the view's datacontext in the code-behind.

Note: this XAML was copied from a working project and simplified for illustration. There may be typos or other minor errors.