ILX provides a way of encoding three features found in many ML-like programming languages into .NET IL. These are discriminated unions, first-class function values and generics. For more information about ILX please see the Abstract IL SDK which contains an implementation of the extensions, the paper ILX: Extending the .NET Common IL for Functional Language Interoperability, presented at Babel 01 in Florence in September 2001, or contact dsyme@microsoft.com. See also the (somewhat out-of-date) ILX reference manual. 

The samples in the SDK are also a good guide. The SDK assumes familiarity with both .NET IL and languages such as ML, e.g. F# or OCaml.

If you plan on implementating a functional language for .NET and wish to produce typed, verifiable IL then you may find the SDK suitable as a backend for your compiler. Note that the implementation of generics assumes that runtime-types do not affect the results of computation).