Static resource shared in merged dictionaries

Question

I'm currently working on having dictionaries of styles and templates that I can dynamically apply to my application. Before this "new wanted" dynamical behavior, I had several resource dictionaries, one for each styled control, that I merged in the App.xaml:

<Application.Resources>
    <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <ResourceDictionary Source="ColorsDictionary.xaml"/>
            <ResourceDictionary Source="ControlsTemplatesDictionary.xaml"/>
        </ResourceDictionary.MergedDictionaries>
    </ResourceDictionary>
</Application.Resources>

Now, I'd like my application to be styled, so I decided to merge all my previous resources into a new one called "MyFirstTemplates" and to add only this dictionary to the App.xaml.

New dictionary "MyFirstTemplates.xaml":

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">"
    <ResourceDictionary.MergedDictionaries>
        <ResourceDictionary Source="ColorsDictionary.xaml"/>
        <ResourceDictionary Source="ControlsTemplatesDictionary.xaml"/>
    </ResourceDictionary.MergedDictionaries>
</ResourceDictionary>

New App.xaml:

<Application.Resources>
    <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <ResourceDictionary Source="MyFirstTemplates.xaml"/>
        </ResourceDictionary.MergedDictionaries>
        <Style TargetType="{x:Type Window}"/>
    </ResourceDictionary>
</Application.Resources>

Note: The default style for the Window is to correct a bug of WPF 4, see Adding a Merged Dictionary to a Merged Dictionary

Now that I have made this change, I cannot use a color resource from "ColorsDictionary.xaml" as a StaticResource in "ControlsTemplateDictionary.xaml" anymore. If I change back to merging these files in the app.xaml, everything works. To make it work, I have to change these StaticResource for DynamicResource. Do you have any idea why this doesn't work anymore?

Thank you :-)

Answer 1

By moving the dictionaries out of App.xaml the resources from each dictionary aren't in the other's resource tree during loading of MyFirstTemplates.xaml. Your original setup first loaded ColorsDictionary which was then available through App resources to ControlsTemplatesDictionary while it loaded. In your new setup, in order for the color resource to be available in App resources it needs to be loaded through MyFirstTemplates, which in turn requires loading of both dictionaries, which in turn requires access to the color resource... so it's sort of an infinite loop of references that can't be resolved statically. DynamicResource can wait until everything is loaded and then access the color without issue.

To fix either use Dynamic or merge ColorsDictionary directly into ControlsTemplatesDictionary.

Answer 2

Great answer by John explaining why this is happening. So the problem is that when using merged dictionaries within a merged dictionary, the inner dictionaries can't "use" each other as StaticResource.

Basic solutions:

• Use DynamicResource
• Use just a single level of hierarchy from App.xaml when using StaticResource

Both of these solutions have problems. DynamicResource has a performance problem. The 2nd solution limits you on how you organize your XAML resources.

Alternative solution:

I created a small simple program (provided below in GitHub) that will run as a pre-build event and merge XAML files from a folder into one long .XAML file. Well, they need to be with a different extension (.txaml), otherwise they will be compiled.

This allows to structure resources folders and files however you want, without WPF’s limitations. StaticResource and the designer will work always.

The code in GitHub contains a simple solution that contains the merging program. It merges 2 folders into 2 files. One for App.xaml resources and the other for Generic.xaml resources. The .xaml files in a "Controls" project (There's also "Main" project).

Blog post explaining this