WPF用户控件依赖项属性绑定

Question

In my mainwindow I have got an UserControl, which ViewModel has got the Dependency Property "Message", I'm trying to bind the Dependency Property to an Property of the ViewModel of the Main Window, but actually it isn't Working, is there any soulution or is it genarally impossible?

Content of the Main Window:

    <local:MessageLayer>
        <local:MessageLayer.DataContext>
            <local:MessageBoxViewModel Message="{Binding RelativeSource={RelativeSource AncestorType=Window, Mode=FindAncestor}, Path=DataContext.Message}"/>
        </local:MessageLayer.DataContext>
    </local:MessageLayer>

Answer 1

View models should not have DependencyProperty s in them and should certainly not extend the DependencyObject class, because these are UI related classes. I'm sure that you, along with many others have been confused by Microsoft's terribly worded error below:

A 'Binding' can only be set on a DependencyProperty of a DependencyObject .

This is really only referring to the UI element side of Binding s and not the data element side. For data binding data objects, we implement the INotifyPropertyChanged interface instead, which provides similar property change notification functionality to DependencyProperty s.

So, had you set the Window.DataContext to an instance of a view model that implemented INotifyPropertyChanged , with a property declared in it named Message , then your code would have worked just fine:

<local:MessageLayer>
    <local:MessageLayer.DataContext>
        <local:MessageBoxViewModel Message="{Binding RelativeSource={RelativeSource 
            AncestorType=Window, Mode=FindAncestor}, Path=DataContext.Message}"/>
    </local:MessageLayer.DataContext>
</local:MessageLayer>