Question

I can’t seem to be able to add a merged dictionary to a collection of merged dictionaries within XAML.

Theme.xaml

<ResourceDictionary.MergedDictionaries>
    <ResourceDictionary Source="/Mine;component/Themes/Palette.Blue.xaml"/>
    <ResourceDictionary Source="/Mine;component/Themes/Template.xaml"/>
</ResourceDictionary.MergedDictionaries>

Application Resources

<Application.Resources>
    <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <ResourceDictionary Source="/Mine;component/Themes/Theme.xaml"/> 
            <!--
            <ResourceDictionary Source=="/Mine;component/Themes/Palette.Blue.xaml"/>
            <ResourceDictionary Source="/Mine;component/Themes/Template.xaml"/>
            -->
        </ResourceDictionary.MergedDictionaries>
    </ResourceDictionary>
</Application.Resources>

Note: if I put both ResourceDictionaries in the Appication.Resources MergedDictionary (comment out the theme.xaml and uncomment the other two dictionaries) they both load correctly. However, the way our resources are defined, this may mean that quite a few resources will be loaded, and for dynamic loading I’d like to be able to define templates.

Answer 1

This is an optimization bug, see this link

On the creation of every object in
XAML, if a default style is present
(i.e. style w/ a key of Type) that
style should be applied. As you can
imagine there are several performance
optimizations to make that (implied)
lookup a light weight as possible. One
of them is that we don’t look inside
Resource Dictionaries unless they are
flagged as “containing default
Styles”. There is a bug: if all your
default styles are nested in merged
dictionaries three levels deep (or
deeper) the top dictionary does not
get flagged so the search skips it.
The work around is to put a default
Style to something, anything, in the root
Dictionary.

So adding a dummy style to the root dictionary fixes this. Example

<Application.Resources>
    <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <ResourceDictionary Source="/Mine;component/Themes/Theme.xaml"/>
        </ResourceDictionary.MergedDictionaries>
        <!-- Dummy Style, anything you won't use goes -->
        <Style TargetType="{x:Type Rectangle}" />
    </ResourceDictionary>
</Application.Resources>