The type or namespace name ‘Linq’ does not exist in the namespace ‘System.Data’ (are you missing an assembly reference?)

Verify that you are using the right version of .net for the solution of your project.

Right click on the projects and click on properties and make sure that you have .net 3.5 selected.

If you are using the right .net version, the you should be able to use :

using system.linq without an issue.