Using BasedOn Style with static ResourceDictionary in different Assembly - wpf

Question

Background
I have multiple projects in my solution and in one project I've made a static class containing a merged ResourceDictionary, which I use in all my other projects:
namespace GUI.Shared.Resources
{
public static class ResourceFactory
{
public static ResourceDictionary _resources = null;
public static ResourceDictionary Resources
{
get
{
if (_resources == null)
{
_resources = new ResourceDictionary();
_resources.MergedDictionaries.Add(new ResourceDictionary() { Source = new Uri("pack://application:,,,/GUI.Shared;component/Resources/VectorIcons.xaml") });
_resources.MergedDictionaries.Add(new ResourceDictionary() { Source = new Uri("pack://application:,,,/GUI.Shared;component/Resources/ButtonStyles.xaml") });
}
return _resources;
}
}
}
}
With my styles defined in the xaml files ButtonStyles.xaml:
<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
<Style x:Key="SpecialButtonStyle" TargetType="{x:Type Button}">
<Setter Property="Background" Value="Red"/>
</Style>
</ResourceDictionary>
Normally I use the style like this:
<UserControl x:Class="GUI.Modules.TextControl.Views.TextControlView"
x:Name="TextControl"
xmlns:resources="clr-namespace:GUI.Shared.Resources;assembly=GUI.Shared">
<Grid>
<Button Style="{Binding Path=[SpecialButtonStyle], Source={x:Static resources:ResourceFactory.Resources}}">
</Grid>
</UserControl>
Problem
Now I want to extend my style locally in a View. I found out that you do this by using the BasedOn property.
<Button>
<Button.Style>
<Style TargetType="Button" BasedOn="{Binding Path=[SpecialButtonStyle], Source={x:Static resources:ResourceFactory.Resources}}">
</Style>
</Button.Style>
</Button>
Only my binding method is not allowed, giving the error:
A Binding cannot be set on the BasedOn property of type Style. A Binding can only be set on a DependencyProperty of a DependencyObject.
How can I use this style from my ResourceFactory.Resources as a BasedOn style?

Answer 1

The BasedOn property is a regular CLR property, not a dependency property, which means you cannot bind it. Furthermore the indexer syntax among other binding path constructs are only available in the Binding markup extension, not in x:Static, StaticResource or DynamicResource.
You can create your own markup extension based on the x:Static implementation, but that is very complex and the wrong thing to do. Instead, you should think about following a different built-in approach to your issue.
If you want to share resources, why create a static ResourceFactory at all? WPF already supports resource provision with the App.xaml resource dictionary and scoped resources within each FrameworkElement through the Resources property. Create a resource dictionary for your shared resources e.g. like this:
<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:WpfApp1">
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="pack://application:,,,/GUI.Shared;component/Resources/VectorIcons.xaml"/>
<ResourceDictionary Source="pack://application:,,,/GUI.Shared;component/Resources/ButtonStyles.xaml"/>
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
As you can see, this is the same as your ResourceFactory, but easier to use. Include this dictionary into the application resources or any other resources section of any project using a pack URI e.g.:
<UserControl x:Class="GUI.Modules.TextControl.Views.TextControlView"
x:Name="TextControl"
xmlns:resources="clr-namespace:GUI.Shared.Resources;assembly=GUI.Shared">
<UserControl.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="pack://application:,,,/GUI.Shared;component/Resources/SharedResourceDictionary.xaml"/>
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</UserControl.Resources>
<Grid>
<Button>
<Button.Style>
<Style TargetType="Button" BasedOn="{StaticResource SpecialButtonStyle}">
<!-- ...your style definition. -->
</Style>
</Button.Style>
</Button>
</Grid>
</UserControl>
There is no need for Binding here and it is much easier to access resources this way. You can access the resources in child controls, too, because access to resources is scoped to where they are defined.