C# compiling error: 'System.Array' does not contain a definition for 'Select' and no extension method 'Select' accepting a first argument of type 'System.Array' could be found (are you missing a using directive or an assembly reference?)

I'm using VS 2008.

The references are:

System

System.AddIn

System.Data

System.Windows.Forms

System.Xml

and a few SQL Server related references