WPF: Binding a ContextMenu to an MVVM Command

Question

Let's say I have a Window with a property returning a Command (in fact, it's a UserControl with a Command in a ViewModel class, but let's keep things as simple as possible to reproduce the problem).

The following works:

<Window x:Class="Window1" ... x:Name="myWindow">     <Menu>         <MenuItem Command="{Binding MyCommand, ElementName=myWindow}" Header="Test" />     </Menu> </Window> 

But the following does not work.

<Window x:Class="Window1" ... x:Name="myWindow">     <Grid>         <Grid.ContextMenu>             <ContextMenu>                 <MenuItem Command="{Binding MyCommand, ElementName=myWindow}" Header="Test" />             </ContextMenu>                     </Grid.ContextMenu>     </Grid> </Window> 

The error message I get is

System.Windows.Data Error: 4 : Cannot find source for binding with reference 'ElementName=myWindow'. BindingExpression:Path=MyCommand; DataItem=null; target element is 'MenuItem' (Name=''); target property is 'Command' (type 'ICommand')

Why? And how do I fix this? Using the DataContext is not an option, since this problem occurs way down the visual tree where the DataContext already contains the actual data being displayed. I already tried using {RelativeSource FindAncestor, ...} instead, but that yields a similar error message.

Answer 1

The problem is that the ContextMenu it not in the visual tree, so you basically have to tell the Context menu about which data context to use.

Check out this blogpost with a very nice solution of Thomas Levesque.

He creates a class Proxy that inherits Freezable and declares a Data dependency property.

public class BindingProxy : Freezable {     protected override Freezable CreateInstanceCore()     {         return new BindingProxy();     }      public object Data     {         get { return (object)GetValue(DataProperty); }         set { SetValue(DataProperty, value); }     }      public static readonly DependencyProperty DataProperty =         DependencyProperty.Register("Data", typeof(object), typeof(BindingProxy), new UIPropertyMetadata(null)); } 

Then it can be declared in the XAML (on a place in the visual tree where the correct DataContext is known):

<Grid.Resources>     <local:BindingProxy x:Key="Proxy" Data="{Binding}" /> </Grid.Resources> 

And used in the context menu outside the visual tree:

<ContextMenu>     <MenuItem Header="Test" Command="{Binding Source={StaticResource Proxy}, Path=Data.MyCommand}"/> </ContextMenu>