Question

I have a WPF application in PRISM architecture.

I have a user-control (view) that has 2 user-controls inside it.

Here is the idea :

The reason why the ‘Search user-control’ is in its own user-control, is because I plan on reusing it in my application.

The main user-control (black) is called ‘MainView’ and has 2 user-controls in it:
– A user-control that has several textbox control I can filter by, and a ‘Search’ button
– A user-control that has a grid where I display the results

Each of these user-controls (and the parent, “Main View”) are ‘Views’ and have ‘View-Models’.

Everything is shown fine, except for the fact that the ‘child’ user-controls do not create their own ‘View-Models’ automatically.

In the view-model’s classes I have the ‘Export’ attribute,
and in the View’s code-behind I have this snippet:

[Import]
FilterFieldsViewModel ViewModel
{           
    set
    {
       this.DataContext = value;
    }
}

Setting the ‘View-Model’ does happen for the ‘Main View’, but not for the child views…

I have a command binded to the ‘Search’ button of the ‘Search User-control’,
and when the application loads – I get an error in the output window, saying :

BindingExpression path error: ‘SearchCommand’ property not found on ‘object’ ”MainWindowViewModel’ (HashCode=22047425)’. BindingExpression:Path=SearchCommand; DataItem=’MainWindowViewModel’ (HashCode=22047425); target element is ‘Button’ (Name=’Search’); target property is ‘Command’ (type ‘ICommand’)

For some reason the command from the ‘child’ user-control tries to bind to the view-model in the main window’s view model, because for some reason – the child user-control’s view model is not instantiated.

Why is this ?

Answer 1

Your dependency injection container (MEF) is not registering imports on the sub-views, because they are created by the MainView and not by MEF. What you probably want to do is create a property on your MainViewModel:

[Import]
public FilterFieldsViewModel FilterFieldsViewModel {get; set; }

Then in XAML:

<UserControl ....>
   ...
   <SearchUserControl DataContext="{Binding FilterFieldsViewModel}" ... />
   ...
</UserControl>

This enables MEF to create an instance of FilterFieldsViewModel for you and sends it down to the SearchUserControl.

This is the basic concept of course, maybe you would want to create a ‘SearchViewModelBase‘ or something that has the FilterFieldsViewModel.