Windows phone 7 Windows phone 7应用程序栏显示为灰色框

我在默认的应用程序栏上做了非常轻微的修改，但是当我运行代码时，它只是显示为一个灰色框。以下是XAML：

<phone:PhoneApplicationPage 
    x:Class="IHeartWords.MainPage"
    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"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"

    mc:Ignorable="d" d:DesignWidth="480" d:DesignHeight="696"
    FontFamily="{StaticResource PhoneFontFamilyNormal}"
    FontSize="{StaticResource PhoneFontSizeNormal}"
    Foreground="{StaticResource PhoneForegroundBrush}"
    SupportedOrientations="Portrait" Orientation="Portrait"
    shell:SystemTray.IsVisible="True" xmlns:my="clr-namespace:Microsoft.Advertising.Mobile.UI;assembly=Microsoft.Advertising.Mobile.UI" Loaded="PhoneApplicationPage_Loaded">

    <!--Sample code showing usage of ApplicationBar-->
    <phone:PhoneApplicationPage.ApplicationBar>
        <shell:ApplicationBar IsVisible="True" IsMenuEnabled="False">
            <shell:ApplicationBarIconButton x:Name="help" IconUri="/icons/appbar.questionmark.rest.png" Text="Help" Click="helpButton_Click"/>
            <shell:ApplicationBarIconButton x:Name="rules" IconUri="/icons/appbar.feature.settings.rest.png" Text="Rules" Click="rulesButton_Click"/>
        </shell:ApplicationBar>
    </phone:PhoneApplicationPage.ApplicationBar>

</phone:PhoneApplicationPage>

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我想出来了。我在代码隐藏中创建了一个新的应用程序条。删除了那个代码，我的XAML就可以了