Textbox - horizontal text centering

Set the TextAlignment property to Center:

<TextBox Width="200"
         Text="Hello world !"
         TextAlignment="Center"/>

HorizontalContentAlignment="Center" VerticalContentAlignment="Center"

You can reach the text within a WPF-TextBox with the combination VerticalAlignment and VerticalContentAlignment. You set the content to center and the total height with Stretch to the size of the comprehensive element like a grid row

<TextBox VerticalAlignment="Stretch" VerticalContentAlignment="Center"> 
Test 
</TextBox>

it's too late but this may be helpful for someone

Try adding this two peoperties to your control

VerticalAlignment="Stretch" 
VerticalContentAlignment="Center"