In Windows phone, how can I select all text in Textbox when the TextBox has focus?

I try setting the get focus property of Textbox:

```
    private void TextBox_GotFocus(object sender, RoutedEventArgs e)
    {
        TextBox textBox = (TextBox)sender;

        textBox .SelectAll();
    }

```

What I see is I see all the text is being selected for 1-2 sec and then it goes back to cursor mode (i.e. 1 blink line).