Writing LINQ Providers will become an essential skill for framework developers over the coming years. In the future all .NET libraries that expose a data store will be expected to have LINQ support, and with the recent release of .NET 3.5, the clock has started ticking.
To make it easier for developers, and especially provider writers, to understand how LINQ works, Bart De Smet has create a LINQ clone called LINQSQO or LINQ Standard Query Operators. He describes it as,
This project was one of my first LINQ-related ones and goes back to the early PDC 05 days where LINQ was first shown to the world. The project contains a custom implementation of the LINQ Standard Query Operators used in LINQ. More specifically, it's a clone of the Enumerable static class and all its extensions methods you can find in the System.Linq namespace in System.Core.dll. Thus, you can see it as an equivalent of LINQ to Objects. The primary goal of the project is to give developers a better insight about how LINQ works. Together with the sources we ship a total of 148 unit tests to do some basic testing of all the query operators in there.
Bart has released the RTM version of LINQSQO on CodePlex.