IDataErrorInfo interface is really handy when implementing data validation in WPF. There are many different ways to implement IDataErrorInfo for a DataContext. But since I've recently become quite fond of FluentValidation library for implementing validators, I'm going to focus on using it in this post.
Having a parameter of type List is not recommended; IList or even IEnumerable should be used instead when a parameter is only going to be used for iterating through the list of items. This way different types of collections, such as arrays, lists, etc. can be passed to it. But IEnumerable can prove even more useful in certain scenarios.
FluentValidation is a small portable validation library with fluent interface. The fluent API makes validators easy to write and understand. You can find a lot of information about basic usage and even more advanced scenarios in very detailed documentation. I'll rather focus on extending the library with your own custom validation.