Exception during applying a style

I'm sure I'm struggling with something very basic, missing a very simple point... But I just keep hitting the wall, so please help.

The problem is as follows.

If I define a certain style in the <Window.Resources> of the same Window where I apply it to controls - everything works just fine. But since I want to reuse this style in a different window of my application, I tried to move the style out to some common location... And here the things stopped working.

If I put the style in Themes\Generic.开发者_C百科xaml, the style doesn't get applied.

When I try to reference it from the original window by explicitly applying Style="{DynamicResource MyStyle}" on the corresponding control (where "MyStyle" is the x:Key of the style) - I get an error

The resource 'MyStyle' could not be resolved

If I put the style in a separate XAML file, and try to add it to MergedDictionaries in my App.xaml, I get a different problem: "'Resources' property has already been set on 'App'". This is how I try to define it:

<Application.Resources>
    <ResourceDictionary x:Key="MergedDictionaries">
        <ResourceDictionary.MergedDictionaries>
            <ResourceDictionary Source="Themes/MyStyle.xaml"/>
        </ResourceDictionary.MergedDictionaries>
    </ResourceDictionary>
<Application.Resources>

---

You should not set the x:Key on the dictionary, otherwise it will be added as a resource which is not what you want, it should set the Application.Resources property instead.

<Application.Resources>
    <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <!-- Merged dictionaries -->
        </ResourceDictionary.MergedDictionaries>

        <!-- Other Resources -->
    </ResourceDictionary>
</Application.Resources>