Depending on the platform version you are using, there are a couple of ways to do this. In Silverlight 5, you can use the Click Count property in the Mouse event args to determine whether the click is a single, double, triple, etc.
In Silverlight 4 however you will have to implement your own logic to determine when a double click has occurred for the GridView.
You can also look into our online example about click events in the GridView.
Hope this helps!
the Telerik team
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