xaml:

<Window.Resources>
        <ViewModel:MainWindowViewModel  
         Canvas="{Binding DataContext.Canvas,Source={x:Reference Name=Canvas}}"  x:Key="KeyMainWindowVIewModel">
        </ViewModel:MainWindowViewModel>
    </Window.Resources>


class viewmodel
public Canvas Canvas
        {
            get { return (Canvas)GetValue(CanvasProperty); }
            set { SetValue(CanvasProperty, value); }
        }

        public static readonly DependencyProperty CanvasProperty =
            DependencyProperty.Register("Canvas", typeof(Canvas), typeof(MainWindowViewModel));
// 

code của mình như thế..Mọi người fix gium mình với . cảm ơn ạ