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John,
Checking uniqueness takes a little bit more thought than simply putting a uniquness constraint on the property, unless you are perfectly happy with an exception if you are trying to save a duplicate. If that's the case, then stop reading. If you however want to alert the user before it hits the database, then read on.
Checking if a property is unique requires querying the DB. In Silverlight that is an asynchronous operation. In WPF it should be an asynchronous operation if you want a good performing application. The proper approach is a validation service that the ViewModel can asynchronously call before saving the data. There's still the possiblity that you get a constraint violation in the DB if an other user snuck in a save between you checking for uniquness and saving the data, so you should still have a DB constraint.
You can find more information and an example here:
http://drc.ideablade.com/xwiki/bin/view/Documentation/unit-of-work-pattern-and-persistence-ignorance#HDomainService - http://drc.ideablade.com/xwiki/bin/view/Documentation/unit-of-work-pattern-and-persistence-ignorance#HDomainService
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