Question

I have the following (condensed) silverlight xaml for a view / viewmodel:

<UserControl x:Class=MyView>    
    <UserControl.Resources>
        <MyViewModel x:Name="MyViewModel"/>
    </UserControl.Resources>

    <Grid DataContext="{Binding Source={StaticResource MyViewModel}}">
<UserControl>

Hopefully this looks familiar to you all.

However, I would like to create 2 instances of this same view user control, but to pass in a parameter to the view model to allow me to have slightly different view model data based on a property which I pass into the view model. Something like:

<UserControl x:Class=MyView>    
    <UserControl.Resources>
        <MyViewModel x:Name="MyViewModel" Filter="Some value set at a higher level"/>
    </UserControl.Resources>

    <Grid DataContext="{Binding Source={StaticResource MyViewModel}}">
<UserControl>

The problem is that I can’t hard code the Filter parameter inside the user control, but need to set it at a higher level. Is there a way to obtain the filter parameter from higher up via binding, and what would the syntax look like. I was hoping something like the following:

Either directly from the parent something like:

<MyView>
    <MyView.ViewModel Filter="All">
</MyView>
<MyView>
    <MyView.ViewModel Filter="Some">
</MyView>

Or from user control looking upwards, something like:

<UserControl.Resources>
    <MyViewModel x:Name="MyViewModel" Filter="{Binding FilterTypeFromDataContextHigherUpTheTree}"/>
</UserControl.Resources>

but I don’t know if is is possible to directly refer to the static resource view model from the parent in order to set a property, or what the syntax would look like.

I also don’t know if there is an easier way to do this, as I suspect my approach is not very elegant.

The real question is how can I pass a parameter into a view model which is a static resourd

Answer 1

Though it’s possible to store the ViewModel as a resource, we usually see the ViewModel set as the DataContext of a view. Then, if someone needs to access the ViewModel through the View, they just cast the DataContext to the proper ViewModel type and access it’s properties directly.

If you want to go the Binding route for your Filter you have a couple of choices. You can set the Bnding Soure to RelativeSource.TemplatedParent if the child control is placed inside of the parent control in a template (or Style). You could also use ElementName, but only if the named element is inside of the same scope (in your example above, the named element would have to be somewhere inside of MyView).

The final option I can think of would be to expose a DependencyProperty for the filter on MyView and then set the binding of the ViewModel to that property. This would effectively bubble the filter so that it’s accessible outside of MyView, but I don’t like this approach at all because it’s adding properties to the View just for the sake of passing them to the ViewModel. That should never happen. The ViewModel should always be accessible independantly of the View, which is why I recommend exposing it through the DataContext property (inferred) or through a custom property specifically for the ViewModel (explicit).