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:

 

 

To add a unique constraint or index to the database table you can write a custom DbInitializer and create the index within the Seed method logic, or as of EF 4.3 use Db migrations.   Here's some more information from StackOverflow - http://stackoverflow.com/questions/4413084/unique-constraint-in-entity-framework-code-first and http://stackoverflow.com/questions/8262590/entity-framework-code-first-fluent-api-adding-indexes-to-columns.

 

You can also write a custom DevForce verifier to validate the property when it's set or the entity is saved - http://drc.ideablade.com/xwiki/bin/view/Documentation/validation-create-custom-verifier.