Custom attached property not found on resource when embedded in user control - wpf

Question

I have a resource that need to be a different color depending on where it is used, so I use this attached property:
public static class AssetProperties
{
public static Brush GetFillBrush(DependencyObject obj)
{
return (Brush)obj.GetValue(FillBrushProperty);
}
public static void SetFillBrush(DependencyObject obj, Brush value)
{
obj.SetValue(FillBrushProperty, value);
}
public static readonly DependencyProperty FillBrushProperty =
DependencyProperty.RegisterAttached("FillBrush",
typeof(Brush),
typeof(AssetProperties),
new FrameworkPropertyMetadata(new BrushConverter().ConvertFrom("#FFE41300"), FrameworkPropertyMetadataOptions.Inherits));
}
We define the symbol and use it something like this in a window or user control (this is of course a lot simplified, the resource is for example defined in a separate file) :
<Grid>
<Grid.Resources>
<ResourceDictionary>
<Rectangle x:Key="SomeColorfulSymbol" x:Shared="False" Width="10" Height="10"
Fill="{Binding (main:AssetProperties.FillBrush), RelativeSource={RelativeSource Self}}" />
</ResourceDictionary>
</Grid.Resources>
<ContentControl Content="{StaticResource SomeColorfulSymbol}" main:AssetProperties.FillBrush="Blue"/>
</Grid>
This works as intended, a nice blue rectangle appears. Without setting the attached property, the rectangle is the default red of the FillBrush attached property.
The problem is when we try to use the symbol inside a custom user control defined like this:
OuterControl.xaml:
<UserControl x:Class="AttachedPropertyResourceTest.OuterControl"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
mc:Ignorable="d"
d:DesignHeight="300" d:DesignWidth="300">
<Grid>
<StackPanel>
<TextBlock Text="Some title"/>
<ContentControl Content="{Binding InnerContent, RelativeSource={RelativeSource FindAncestor, AncestorType=UserControl}}"/>
</StackPanel>
</Grid>
</UserControl>
OuterControl.xaml.cs:
[ContentProperty("InnerContent")]
public partial class OuterControl
{
public FrameworkElement InnerContent
{
get { return (FrameworkElement)GetValue(InnerContentProperty); }
set { SetValue(InnerContentProperty, value); }
}
public static readonly DependencyProperty InnerContentProperty =
DependencyProperty.Register("InnerContent", typeof(FrameworkElement), typeof(OuterControl), new FrameworkPropertyMetadata(null));
public OuterControl()
{
InitializeComponent();
}
}
Now if I wrap the ContentControl in the above snippet like this instead:
<main:OuterControl>
<ContentControl Content="{StaticResource SomeColorfulSymbol}"/>
</main:OuterControl>
it looks good in the VS designer, a title plus a rectangle that is the default red of FillBrush. In runtime however we only get the title. The rectangle gets no color (UnsetValue) and we get this binding error:
System.Windows.Data Error: 40 : BindingExpression path error:
'(main:AssetProperties.FillBrush)' property not found on 'object'
''Rectangle' (Name='')'.
BindingExpression:Path=(main:AssetProperties.FillBrush);
DataItem='Rectangle' (Name=''); target element is 'Rectangle'
(Name=''); target property is 'Fill' (type 'Brush')
If I add an invisible instance of the symbol before the wrapped one, it works again, i.e., a red rectangle appears:
<ContentControl Content="{StaticResource SomeColorfulSymbol}" Visibility="Collapsed"/>
<main:OuterControl>
<ContentControl Content="{StaticResource SomeColorfulSymbol}"/>
</main:OuterControl>
One problem is that the attached property is not registered, when I put a breakpoint on the RegisterAttached method it is not called without the extra invisible ContentControl. This is however only a part of the problem, for example forcing the registration like this does not work:
<StackPanel>
<TextBlock Text="I'm red!" Background="{Binding (main:AssetProperties.FillBrush), RelativeSource={RelativeSource Self}}"/>
<main:OuterControl>
<ContentControl Content="{StaticResource SomeColorfulSymbol}"/>
</main:OuterControl>
</StackPanel>
The text "I'm red" is actually red and the attached property is registered, but we get the exact same binding error.
I also tried without the ContentProperty["InnerContent"], setting the InnerContent attribute explicitly in xaml, with the same result.
Could someone shed some light on this?
Maybe using a control template instead of OuterControl wouldn't have this problem (?), but there is a lot of behavior associated with OuterControl and I would prefer this approach.

Answer 1

To prevent the following issue, try specifying the path property explicitly like:{Binding Path=(main:....}
<Rectangle x:Key="SomeColorfulSymbol" x:Shared="False" Width="10" Height="10" Fill="{Binding Path=(main:AssetProperties.FillBrush), RelativeSource={RelativeSource Self}}" />