LINQBridge is a re-implementation of all the standard query operators in Framework 3.5's System.Linq.Enumerable class. It's designed to work with the C# 3.0 compiler, as used by Visual Studio 2008. LINQBridge comprises a LINQ to Objects API for running local queries. (It doesn't include an implementation of LINQ to SQL, nor LINQ to XML; a good compromise can be to force Framework 3.5 out to just the server machines, allowing LINQ to SQL to be used where it's needed most).

via linqbridge - Re-implementation of LINQ to Objects for .NET Framework 2.0

There are a lot of legacy project out of there. Some customer will ask you to add a simple .NET 2.0 web service to a big application, and they will not switch to C# 3.5 for that.

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