I try to help you by explanation how it works in my projects.
A separate assembly contains a common control and common resource dictionary like this:
CommonResources.xaml
<ResourceDictionary
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
<!-- Some resources -->
</ResourceDictionary>
SomeCommonControl.xaml
<UserControl x:Class="YourAssembly.SomeCommonControl"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
<UserControl.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="pack://application:,,,/YourAssembly;component/CommonResources.xaml" />
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</UserControl.Resources>
<!-- Some specific content -->
</UserControl>
I can use this control and resources from another assemblies and WPF-projects like this:
<Window x:Class="YourWPFProject.SomeWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:common="clr-namespace:YourAssembly">
<Window.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="pack://application:,,,/YourAssembly;component/CommonResources.xaml" />
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</Window.Resources>
<common:SomeCommonControl />
</Window>
I hope this will help you.