I'm trying to better understand the MergeDictionaries functionality in order to resolve design time errors from displaying in the XAML. We have an application "Theme" which is basically made up of a bunch of <UIElement>.xaml files (e.g. Button.xaml, TextBox.xaml) which are nothing more than ResourceDictionaries. These files usually import other "common" ResourceDictionaries which contain Colors, Fonts, etc and then build on these to include custom Style and Template functionality. We then have a master Theme.xaml file which Merges all of the <UIElement>.xaml files into a MergeDictionary that is then set in code to have Application Scope. Our problem is when we need to override styles in a particular user control, we have a local scoped merge dictionary which imports the Theme.xaml and then creates another ResourceDictionary that attempts to base off of the Application Scope <UIElement>.xaml styling. This causes VS to show errors such as (DependencyProperty.UnsetValue} is not a valid value for X property on a Setter.
This may be easier to explain with a sample:
The XAML file hierarchy looks like this
Theme.xaml (Application Scope)
Button.xaml (UIElement specific Styles)
Common.xaml (Common Styles: Colors, Fonts, etc)
TextBox.xaml
Common.xaml
<!--Button.xaml-->
<ResourceDictionary> <ResourceDictionary.MergedDictionaries> <ResourceDictionary Source="Common.xaml"/> </ResourceDictionary.MergedDictionaries> <!--Standard Button--> <Style x:Key="{x:Type Button}" TargetType="{x:Type Button}"> <Setter Property="Background" Value="{StaticResource ButtonBackground}"/> </Style></ResourceDictionary>
<!--UserControl1.xaml-->
<UserControl.Resources> <ResourceDictionary> <ResourceDictionary.MergedDictionaries> <ResourceDictionary Source="X/Shared;component/WPFThemes/Theme.xaml" /> </ResourceDictionary.MergedDictionaries> <ResourceDictionary**> <Style x:Key="OverrideTextBox" TargetType="{x:Type TextBox}" BasedOn="{StaticResource {x:Type TextBox}}"> <Setter Property="Width" Value="80" /> </Style> </ResourceDictionary**> </ResourceDictionary.MergedDictionaries> </ResourceDictionary></UserControl.Resources>Now, I know that you aren't supposed to reference resources across Resource Dictionaries that are merged together, which is what the code above is doing (It resolves as expected at runtime, but a pain at design time). If I remove the Local Scope ResourceDictionary (marked with the **) leaving the inner Style, this is still a problem. If I move the Style into the top Resource Dictionary as a child, everything operates as expected, but I've not seen anyone do this in all the examples I've looked at. As I understand it when you Source a ResourceDictionary that is the only thing should be included (i.e. no child elements). Suggestions?