Import multiple resource files in app.xaml wpf

Question

[Running on Windows 8] I know how to import a single one, like this:

<Application.Resources>
        <ResourceDictionary Source="Styles\MyCombobox.xaml"/>       
 </Application.Resources> 

but I want something like this:

<Application.Resources>
        <ResourceDictionary Source="Styles\X.xaml"/>       
        <ResourceDictionary Source="Styles\Y.xaml"/>       
        <ResourceDictionary Source="Styles\Z.xaml"/>       
 </Application.Resources> 

I get:

The property Resource can only be set once!

Answer 1

You should merge that dictionaries.

<Application.Resources>
    <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <ResourceDictionary Source="Styles\X.xaml"/>       
            <ResourceDictionary Source="Styles\Y.xaml"/>       
            <ResourceDictionary Source="Styles\Z.xaml"/>       
        </ResourceDictionary.MergedDictionaries>
    </ResourceDictionary>
</Application.Resources>

More in documentation

UPDATE

Of course, you should wrap property usage in ResourceDictionary tag.