Adding resource dictionaries to a usercontrol library in wpf

Just to clearify the answer of @Willem: when having additional resources after the merged dictionary, there may appear the error "x:key attribute is required". In this case, those resources have to be inside the resource dictionary:

 <UserControl.Resources>
    <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <ResourceDictionary source="/assembly;component/resource.xaml" />
        </ResourceDictionary.MergedDictionaries>
      
        <Style TargetType="TextBlock" BasedOn="{StaticResource SetupTextBlockStyle}" />

    <ResourceDictionary>
 </UserControl.Resources>

 <Grid />

Give this a try:

<UserControl.Resources>
    <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <ResourceDictionary Source="pack://application:,,,/{YourAssemblyWhereResourceDictionaryIsLocated};component/Resources/MoveResizeThumb.xaml" />
        </ResourceDictionary.MergedDictionaries>
    </ResourceDictionary>
</UserControl.Resources>