如何在网格内居中放置TextBox元素

我正在为自己的需要开发WP8应用程序，并希望它具有带文本的小型实时显示块。由于小图块无法显示文本，因此我正在生成带有所需文本的适当图像。这是代码：

WriteableBitmap bmpSmall = new WriteableBitmap(159, 159);
var grid = new Grid();
grid.Width = bmpSmall.PixelWidth;
grid.Height = bmpSmall.PixelHeight;

var background = new Canvas();
background.Width = bmpSmall.PixelWidth;
background.Height = bmpSmall.PixelHeight;

SolidColorBrush backColor = new SolidColorBrush((Color)Application.Current.Resources["PhoneAccentColor"]);
background.Background = backColor;

var textBlock = new TextBlock();
textBlock.Text = "qwerty";
textBlock.FontWeight = FontWeights.Bold;
textBlock.HorizontalAlignment = HorizontalAlignment.Center;
textBlock.VerticalAlignment = VerticalAlignment.Center;
textBlock.FontSize = 28;
textBlock.Foreground = new SolidColorBrush(Colors.White);

grid.Children.Add(textBlock);
bmpSmall.Render(background, null);
bmpSmall.Render(grid, null);
bmpSmall.Invalidate();

using (IsolatedStorageFile isf = IsolatedStorageFile.GetUserStoreForApplication())
{
    using (IsolatedStorageFileStream imageStream = new IsolatedStorageFileStream("/Shared/ShellContent/smallTile.jpg", System.IO.FileMode.Create, isf))
    {
        bmpSmall.SaveJpeg(imageStream, 159, 159, 0, 100);
    }
}

ShellTile tile = ShellTile.ActiveTiles.First();
FlipTileData tileData = new FlipTileData();
tileData.SmallBackgroundImage = new Uri(@"isostore:/Shared/ShellContent/smallTile.jpg", UriKind.Absolute);
tile.Update(tileData);

结果看起来像：

如您所见，文本与左上角对齐。问题是“为什么”？既然你可以设置textBlock.HorizontalAlignment和textBlock.VerticalAlignment-我希望它在图像的中心。例如，以下XAML看起来像您期望的一样，就像我需要的：

<Grid Width="159" Height="159">
    <Grid.Background>
        <SolidColorBrush Color="{StaticResource PhoneAccentColor}"/>
    </Grid.Background>
    <TextBlock HorizontalAlignment="Center" VerticalAlignment="Center" FontWeight="Bold" FontSize="28" Foreground="White">qwerty</TextBlock>
</Grid>

我错过了什么？如何将文字居中？