WPF Context menu doesn't bind to right databou

问题:

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; }
}

回答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}}}"
/>

回答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.

回答3:

I had the same issue recently with a ContextMenu located in a ListBox. I tried to bind a command the MVVM way without any code-behind. I finally gave up and I asked a friend for his help. He found a slightly twisted but concise solution. He is passing the ListBox in the DataContext of the ContextMenu and then find the command in the view model by accessing the DataContext of the ListBox. This is the simplest solution that I have seen so far. No custom code, no Tag, just pure XAML and MVVM.

I posted a fully working sample on Github. Here is an excerpt of the XAML.

<Window x:Class="WpfListContextMenu.MainWindow" 
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        Title="MainWindow" Height="350" Width="268">
  <Grid>
    <DockPanel>
      <ListBox x:Name="listBox" DockPanel.Dock="Top" ItemsSource="{Binding Items}" DisplayMemberPath="Name"
               SelectionMode="Extended">
        <ListBox.ContextMenu>
          <ContextMenu DataContext="{Binding Path=PlacementTarget, RelativeSource={RelativeSource Self}}">
            <MenuItem Header="Show Selected" Command="{Binding Path=DataContext.ShowSelectedCommand}"
                      CommandParameter="{Binding Path=SelectedItems}" />
          </ContextMenu>
        </ListBox.ContextMenu>
      </ListBox>
    </DockPanel>
  </Grid>
</Window>

回答4:

Check out: ContextMenuServiceExtensions.DataContext

回答5:

I prefer another solution. Add context menu loader event.

<ContextMenu Loaded="ContextMenu_Loaded"> 
    <MenuItem Header="Change" Command="{Binding Path=ChangeCommand}"/> 
</ContextMenu> 

Assign data context within the event.

private void ContextMenu_Loaded(object sender, RoutedEventArgs e)
{
    (sender as ContextMenu).DataContext = this; //assignment can be replaced with desired data context
}

回答6:

I found this method using the Tag property very useful when binding from a context menu deep inside a control template:

http://blog.jtango.net/binding-to-a-menuitem-in-a-wpf-context-menu

This makes it possible to bind to any datacontext available to the control that the context menu was opened from. The context menu can access the clicked control through "PlacementTarget". If the Tag property of the clicked control is bound to a desired datacontext, binding to "PlacementTarget.Tag" from inside the context menu will slingshot you directly to that datacontext.

回答7:

I know this is already an old post, but I would like to add another solution for those one who are looking for different ways to do it.

I could not make the same solution to work in my case, since I was trying to do something else: open the context menu with a mouse click (just like a toolbar with a submenu attached to it) and also bind commands to my model. Since I was using an Event Trigger, the PlacementTarget object was null.

This is the solution I found to make it work only using XAML:

<!-- This is an example with a button, but could be other control -->
<Button>
  <...>

  <!-- This opens the context menu and binds the data context to it -->
  <Button.Triggers>
    <EventTrigger RoutedEvent="Button.Click">
      <EventTrigger.Actions>
        <BeginStoryboard>
          <Storyboard>
            <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="ContextMenu.DataContext">
              <DiscreteObjectKeyFrame KeyTime="0:0:0" Value="{Binding}"/>
            </ObjectAnimationUsingKeyFrames>
            <BooleanAnimationUsingKeyFrames Storyboard.TargetProperty="ContextMenu.IsOpen">
              <DiscreteBooleanKeyFrame KeyTime="0:0:0" Value="True"/>
            </BooleanAnimationUsingKeyFrames>
          </Storyboard>
        </BeginStoryboard>
      </EventTrigger.Actions>
    </EventTrigger>
  </Button.Triggers>

  <!-- Here it goes the context menu -->
  <Button.ContextMenu>
    <ContextMenu>
      <MenuItem Header="Item 1" Command="{Binding MyCommand1}"/>
      <MenuItem Header="Item 2" Command="{Binding MyCommand2}"/>
    </ContextMenu>
  </Button.ContextMenu>

</Button>