Data validation in WPF, my approach

Whether to use IDataErrorInfo or ValidationRule – that is the question, as I posed in a previous post.

So , you may be wondering what do I do in my WPF projects?
Or you may not Image may be NSFW.
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Well, I tend to use both!
I use IDataErrorInfo to provide validation in the business object itself, then derive extra classes from ValidationRule when I want validation across different business objects.

I have tried making the Validation classes always separate from the pure data business classes, but I found it to be counter productive and also counter intuitive.