Variance and Generalized Constraints, MS Research Publications

A very good document with recommendation for Variance and Generalized Constraints implementation in C# by Burak Emir, Andrew J. Kennedy, Claudio Russo, and Dachuan Yu.

Let me just quote the Introduction so you can understand what it is all about:

The Generics feature of C# 2.0 introduced parametric polymorphism to the language, supporting type parameterization for types (classes, interfaces, structs,and delegates) and methods (static and instance). Being object-oriented, C# already offers subtype polymorphism, namely the ability for a value of type T to be used in a context that expects type U, if T is a subtype of U.
As it stands, though, subtype and parametric polymorphism interact only through subclassing. In particular, there is no subtyping relationship between distinct instantiations of the same generic type type parameters are said to behave invariantly with respect to subtyping. This leads to a certain inflexibility: a method whose parameter has type IEnumerable<Control>cannot be passed an argument of type IEnumerable<Button>,even though this is safe: since Button is a subclass of Control, something that enumerates Buttons also enumerates Controls. Dually, a method expecting a parameter of type IComparer<Button> cannot be passed an argument of type IComparer<Control>, even though this is safe: something that can compare Controls can also compare Buttons.

The document then details possible extensions to C# in order to express Variance and Generalized Constraints.

I hope MS will implement the recommendations since I already found myself dealing with this issue in the past and I’m not entirely happy with the workaround I used.