title: Binding issue after data context changed
body: <p>I have a strange problem: I have a WPF screen on which I have an extended <code>TextBox</code> that makes some conversions in some cases and that property is bound to a property on view model. When the data context is changed the conversions are reevaluated. When reevaluated my extended <code>TextBox</code> has the new data context but the <code>BindingOperations.GetBindingExpression(textBox, dpproperty).DataItem</code> is still the old data context.
  Maybe the conversion that I make is too early after changind the data context? Usually the data context is not changed into the whole screen so the bindings to be reevaluated?
  Thanks!</p>