Setting focus on Windows Phone PhoneTextBox throws Exception with Message "0x8000ffff"

Question

On my WP7.1 app, Windows Phone Toolkit PhoneTextBox occasionally throws an exception with Message "0x8000ffff" when setting the focus on it (by tapping the PhoneTextBox). The problem is reproduced this way:

1. Ensure that the PhoneTextBox.IsReadOnly is set to true.
2. Press back button to go to the previous page.
3. Navigate back to the page that contains PhoneTextBox.
4. Tap the PhoneTextBox so that it gains focus.

I reported this problem to Windows Phone Toolkit against version 4.2012.10.30: http://phone.codeplex.com/workitem/10726

The exception is not thrown if PhoneTextBox.IsReadOnly is set to false when back button is pressed. Since false is the default value, this exception doesn't occur at all if you don't modify the value of PhoneTextBox.IsReadOnly property.

Answer 1

Before this is fixed in WP Toolkit, a workaround is to set the PhoneTextBox.IsReadOnly to false in OnNavigatingFrom event handler (doing it in OnNavigatedTo doesn't work).

    protected override void OnNavigatingFrom(NavigatingCancelEventArgs e)
    {
        ProblematicPhoneTextBox.IsReadOnly = false;
        base.OnNavigatingFrom(e);
    }