Last week Bart De Smet posted an excellent explanation of co-variance and contra-variance support in the .NET CLR and the new support for this in C# 4.0 that can make our lives easier.

This is by far the most comprehensive and clear explanation of the rather in-depth field of type variance, and features lots of examples (and diagrams!) to help your understanding.

My favourite part of the article was this awesome fruit farmer metaphor:

 "This might go unnoticed if the farmer doesn’t enforce runtime fruit/vegetable type safety."
   — Bart De Smet 

Read the full article here:

C# 4.0 FEATURE FOCUS – PART 4 – CO- AND CONTRA-VARIANCE FOR GENERIC DELEGATE AND INTERFACE TYPES