WPF Get Multibinding Expression， Update Source，

wpf 拿到某个control的multibinding以及其中每个Binding

1. 拿到multibinding

      MultiBindingExpression  mbe = BindingOperations . GetMultiBindingExpression (( child as TextBox ), TextBox . TextProperty );

2. 拿到其中每个Binding的Path

        Binding bd = mbe . ParentMultiBinding . Bindings [0] as Binding ;

                bindingPath = bd . Path . Path ;

3. 拿到其中的ValidationRule

      ValidationRule  vr = mbe . ParentMultiBinding . ValidationRules [0];

4. 更新MultiBinding

        mbe . UpdateSource ();