我的用户控件：
XAML：

<UserControl x:Name="LoadingProgressBarPage"
x:Class="Project1.Custom.General.UserControls.LoadingOverlayFullScreen"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:Merakyahoga.com.Custom.General.UserControls"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
 mc:Ignorable="d"
FontFamily="{StaticResource PhoneFontFamilyNormal}"
 
Foreground="{x:Null}"
d:DesignHeight="800" d:DesignWidth="480" >
 
<Grid x:Name="LayoutRoot">
    <Grid.RowDefinitions>
        <RowDefinition Height="1*"/>
        <RowDefinition Height="1*"/>
        <RowDefinition Height="1*"/>
    </Grid.RowDefinitions>
    <StackPanel 
        Name="LayoutRoot_StackPanelMain"
        Grid.Row="1"
                Orientation="Vertical">
        <ProgressBar Name="ProgressBar" 
                     IsIndeterminate="True"
                     Foreground="{StaticResource DefaultTheme_DarkBlueColor}" 
 
                     Height="50" 
                     Width="480" 
                     VerticalAlignment="Center"/>
 
        <StackPanel Name="LayoutRoot_SubStackPanel"
                    Orientation="Horizontal"
                    HorizontalAlignment="Center"
                    VerticalAlignment="Center">
 
            <Image Name="ImageHolder_Main"
                   Width="54">    
            </Image>
 
            <TextBlock Name="ProgressBarText"
                   Text="Please Wait.." 
                       Margin="10,0"
                   FontWeight="Normal"
                   HorizontalAlignment="Center"
                       FontSize="18"
                       Foreground="{StaticResource DefaultTheme_DarkBlueColor}" VerticalAlignment="Center"/>
 
        </StackPanel>
 
    </StackPanel>
</Grid>

CS：

namespace Project1.Custom.General.UserControls
{
    public partial class LoadingOverlayFullScreen : UserControl
    {
 
 
        public LoadingOverlayFullScreen()      
        {
            InitializeComponent()
 
            //WINRT:
            this.LayoutRoot.Height = Window.Current.Bounds.Height;
            this.LayoutRoot.Width = Window.Current.Bounds.Width;
 
        }
 
    }
 
}

在我的mainpage.cs中：

LoadingOverlayFullScreen LoadingOverlayFullScreenObject = new LoadingOverlayFullScreen();
        //test:
LoadingOverlayFullScreenObject.ProgressBarText = "Hello"; //ERROR Here >> **element inaccessible due to its protection level**