可附属性“The medicalaries”;在“资源词典”类型中找不到

以下代码位于Windows Phone 8应用程序的app.xaml中。它接收到错误The attachable property "ThemeDictionaries" was not found in type "ResourceDictionary"。同样，它也不设置背景。我试图在整个应用程序中设置背景，并通过c#代码使用(App.Current.Resources["BackgroundImage"] as ImageBrush).ImageSource = imgSrc;更改它。

<Application
    x:Class="MyApp.App"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:phone="clr-namespace:Microsoft.Phone.Controls;assembly=Microsoft.Phone"
    xmlns:shell="clr-namespace:Microsoft.Phone.Shell;assembly=Microsoft.Phone">  
     <Application.Resources>
          <ResourceDictionary>
           <ResourceDictionary.ThemeDictionaries>
              <ResourceDictionary x:Key="Default">
                    <ImageBrush x:Key="BackgroundImage" ImageSource="Assets/BackgroundDefault.jpg" Stretch="UniformToFill" />
              </ResourceDictionary>
              <ResourceDictionary x:Key="Dark">
                    <ImageBrush x:Key="BackgroundImage" ImageSource="Assets/BackgroundDark.jpg" Stretch="UniformToFill" />
              </ResourceDictionary>
           </ResourceDictionary.ThemeDictionaries>
            <local:LocalizedStrings xmlns:local="clr-namespace:MyApp" x:Key="LocalizedStrings"/>
            <DataTemplate x:Key="SmallPanoramaTitle">
                <ContentPresenter>
                    <TextBlock Text="{Binding}" FontSize="25" Margin="0,30,0,0" />
                </ContentPresenter>
            </DataTemplate>
        </ResourceDictionary>
      </Application.Resources>
      <Application.ApplicationLifetimeObjects>
        <!--Required object that handles lifetime events for the application-->
        <shell:PhoneApplicationService
            Launching="Application_Launching" Closing="Application_Closing"
            Activated="Application_Activated" Deactivated="Application_Deactivated"/>
       </Application.ApplicationLifetimeObjects>
</Application>