There is what I would consider a bug in the TextBinding attached property helper. Classes that derive from TextBox and PasswordBox should be able to take advantage of this attached property, but they cannot due to the current implementation. In my case, this causes a problem with the PhoneTextBox in the Windows Phone Toolkit.
The offending code is:
private static void HandleUpdateSourceOnChangeEventAppend(object sender, bool value)
var type = sender.GetType();
if (type == typeof(TextBox))
else if (type == typeof(PasswordBox))
Changing the "type == typeof(TextBox)" statements to "sender is TextBox" and "sender is PasswordBox" would resolve this issue without any other side effects that I can see.