WPF TextBox 实现CornerRadius圆角

<Grid Background="Gray" Height="230" Width="229">       

 <Border CornerRadius="15" BorderBrush="White" BorderThickness="5" Margin="12,23,20,174" Background="White">       

    <TextBox Width="175" Height="30" Margin="-5,-3,0,0" BorderBrush="White"/> 

</Border>  

 </Grid>