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();