…Both the VB and C# DynamicQuery samples include a source implementation of a helper library that allows you to express LINQ queries using extension methods that take string arguments instead of type-safe language operators. You can copy/paste either the C# or VB implementations of the DynamicQuery library into your own projects and then use it where appropriate to more dynamically construct LINQ queries based on end-user input.
weblogs.asp.net/scottgu/dynamic-linq-part-1-using-the-linq-dynamic-query-library