Setting a ViewModel Property Value From XAML - wpf

Question

I have a view that is declared in XAML (see below). The associated view-model is created automatically using MEF. I want to be able to do something like this:
<local:MyControl Owner={x:Static local:Owners.ProjectOwner} />
The desired net effect is for some view-model property to be set equal to Owners.ProjectOwner.
I can achieve the required result using hacky code-behind but would rather do this through bindings or some similar manner. Can anyone suggest a way of doing this?
UPDATE
I resigned myself to writing a behaviour. But rather than put in all the effort solely for one specific case, I have genericised my solution and I include it below in case anyone's interested. It's a Blend behaviour (System.Windows.Interactivity.dll) but could just as easily be a conventional attached behaviour.
using System;
using System.Windows;
using System.Windows.Interactivity;
namespace BlendBehaviors
{
public class SetViewModelPropertyBehavior : Behavior<FrameworkElement>
{
public static readonly DependencyProperty PropertyNameProperty =
DependencyProperty.Register("PropertyName", typeof(string), typeof(SetViewModelPropertyBehavior));
public static readonly DependencyProperty PropertyValueProperty =
DependencyProperty.Register("PropertyValue", typeof(object), typeof(SetViewModelPropertyBehavior));
public SetViewModelPropertyBehavior()
{ }
public string PropertyName
{
get { return (string)GetValue(PropertyNameProperty); }
set { SetValue(PropertyNameProperty, value); }
}
public object PropertyValue
{
get { return GetValue(PropertyValueProperty); }
set { SetValue(PropertyValueProperty, value); }
}
protected override void OnAttached()
{
base.OnAttached();
var ao = AssociatedObject;
SetViewModel(ao.DataContext);
ao.DataContextChanged += FrameworkElement_DataContextChanged;
}
protected override void OnDetaching()
{
base.OnDetaching();
AssociatedObject.DataContextChanged -= FrameworkElement_DataContextChanged;
}
private void FrameworkElement_DataContextChanged(object sender, DependencyPropertyChangedEventArgs e)
{
SetViewModel(e.NewValue);
}
private void SetViewModel(object viewModel)
{
SetViewModelProperty(viewModel, PropertyName, PropertyValue);
}
private static void SetViewModelProperty(object viewModel, string propertyName, object propertyValue)
{
if (viewModel == null || propertyName == null) {
return;
}
var info = viewModel.GetType().GetProperty(propertyName);
if (info != null && CanAssignValue(propertyValue, info.PropertyType)) {
info.SetValue(viewModel, propertyValue, null);
}
}
private static bool CanAssignValue(object value, Type targetType)
{
if (value == null) {
return !targetType.IsValueType || Nullable.GetUnderlyingType(targetType) != null;
}
return targetType.IsAssignableFrom(value.GetType());
}
}
}
Then use it like this:
<local:MyControl>
<i:Interaction.Behaviors>
<bb:SetViewModelPropertyBehavior PropertyName="Owner" PropertyValue="{x:Static local:Owners.ProjectOwner}" />
<bb:SetViewModelPropertyBehavior PropertyName="AnotherProperty" PropertyValue="{StaticResource MyResourceKey}" />
</i:Interaction.Behaviors>
</local:MyControl>

Answer 1

The target of any WPF binding must be a DependencyProperty. The source can be a DependencyProperty, a CLR object that implements INotifyPropertyChanged, or just some object. The target and source can be swapped around by altering the Binding.Mode property.
But in this case one of the items in your binding is a statically-resolved property (Owners.ProjectOwner). Therefore, it's not a DependencyProperty. Therefore, it can only appear as a source. Therefore, what you're binding it to (the target) must be a DependencyProperty. Therefore, it cannot be a property on your view model (assuming you've not created DependencyObject-based view models, which would be a mistake).
So, you cannot directly bind a property on your VM to the static property. You could write an attached behavior that does this for you though.