How to center TextBox element inside Grid

How to center TextBox element inside Grid

I'm developing WP8 app for my own needs and want it to have small live
tile with text. Since small tile cannot display text, I'm generating
appropriate image with needed text. Here is the code:
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);
And result looks like:
http://s22.postimg.org/wori0zcg1/wp_ss_20130917_0001.png
As you see, text is aligned to top left corner. The question is "Why"?
Since I'd set textBlock.HorizontalAlignment and
textBlock.VerticalAlignment - I expect it in the center of the image. For
example the following XAML looks like you can expect and like I need:
<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>
What did I miss? How can I center text?