如何在WPF中使用外部资源文件？

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <Image x:Key="Logo" Source="Images/logo.png"/>
    <Style x:Key="ButtonStyle" TargetType="Button">
        <Setter Property="Background" Value="Red"/>
        <Setter Property="Foreground" Value="White"/>
    </Style>
    <sys:String x:Key="AppName">My Application</sys:String>
</ResourceDictionary>

<Window x:Class="MyApplication.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="My Application" Height="450" Width="800">
    <Window.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="Resources.xaml"/>
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Window.Resources>
    
    <!-- 在此处可以使用外部资源文件中定义的资源 -->
    <Grid>
        <Button Style="{StaticResource ButtonStyle}" Content="{StaticResource AppName}"/>
        <Image Source="{StaticResource Logo}"/>
    </Grid>
</Window>