Which class needs to implement INotifyDataErrorInfo?

Question

In the case of a binding such as

<TextBox Text={Binding Path=SomeItem.AnotherItem.Property} />

Which class(es) need to implement INotifyDataErrorInfo:

1. The DataContext
2. SomeItem
3. AnotherItem
4. Some combination of these

Answer 1

AnotherItem

INotifyDataErrorInfo must be implemented by object who own's the property to which is bound.

Answer 2

If you're using an MVVM pattern , the INotifyDataErrorInfo is usually implemented by the view-model. This view-model, usually, is then your view's datacontext.