Question

I have a working attached behavior which I would like to add a DP to. I can set the property in XAML but it is null when I try to access it.

What is the fix?

Cheers,
Berryl

xaml

<Button Command="{Binding ContactCommand}" local:ContactCommandBehavior.ResourceKey="blah" >
    <i:Interaction.Behaviors>
        <local:ContactCommandBehavior />
    </i:Interaction.Behaviors>
</Button>

Behavior code

internal class ContactCommandBehavior : Behavior<ContentControl>
{
    ...

    public static readonly DependencyProperty ResourceKeyProperty = 
        DependencyProperty.RegisterAttached("ResourceKey", typeof(string), typeof(ContactCommandBehavior));

    public static string GetResourceKey(FrameworkElement element)
    {
        return (string)element.GetValue(ResourceKeyProperty);
    }

    public static void SetResourceKey(FrameworkElement element, string value)
    {
        element.SetValue(ResourceKeyProperty, value);
    }

    private void SetProperties(IHaveDisplayName detailVm)
    {

        //************ 
        var key = GetResourceKey(AssociatedObject);
        //************ 
        ....
    }

}

Edit for HighCore.

I change the code as follows, changing RegisterAttached to Register and making the property non-static. The value is still null when I try to get at it though

public static readonly DependencyProperty ResourceKeyProperty =
    DependencyProperty.Register("ResourceKey", typeof (string), typeof (ContactCommandBehavior));

public string ResourceKey
{
    get { return (string)GetValue(ResourceKeyProperty); }
    set { SetValue(ResourceKeyProperty, value); }
}

protected override void OnAttached() {
    base.OnAttached();
    if (AssociatedObject == null)
        throw new InvalidOperationException("AssociatedObject must not be null");

    AssociatedObject.DataContextChanged += OnDataContextChanged;
    CultureManager.UICultureChanged += OnCultureChanged;
}

private void OnDataContextChanged(object sender, DependencyPropertyChangedEventArgs e) {
    // do some setup stuff
    SetProperties(vm)
}

private void SetProperties(IHaveDisplayName detailVm)
{
    ////////////////////////////////
    var key = ResourceKey.Replace(TOKEN, cmType);
    /////////////////////////////////
}

Answer 1

Use a regular DependencyProperty in the Behavior instead of an attached one, then you can do

<Button Command="{Binding ContactCommand}">
    <i:Interaction.Behaviors>
        <local:ContactCommandBehavior ResourceKey="blah"/>
    </i:Interaction.Behaviors>
</Button>

which is a much nicer syntax. Also, make sure the code you try to read these properties only AFTER OnAttached() has ocurred.