As linked to by the C# Futures site, here are the four main groups of C# 4.0 proposed features:

Dynamic lookup

Dynamic lookup allows you to write method, operator and indexer calls, property and field accesses, and even object invocations which bypass the C# static type checking and instead gets resolved at runtime.

Named and optional parameters

Parameters in C# can now be specified as optional by providing a default value for them in a member declaration. When the member is invoked, optional arguments can be omitted. Furthermore, any argument can be passed by parameter name instead of position.

COM specific interop features

Dynamic lookup as well as named and optional parameters both help making programming against COM less painful than today. On top of that, however, we are adding a number of other small features that further improve the interop experience.

Variance

It used to be that an IEnumerable<string> wasn’t an IEnumerable<object>. Now it is – C# embraces type safe “co-and contravariance” and common BCL types are updated to take advantage of that.

Most of these features are similar in spirit to what was shown at the Lang.NET symposium, but with a few details worked out.

The Dynamic features are going to be great when dealing with objects from DLR languages or COM. Named parameters sounds kind-of nice, but not world-changing. Variance support when assigning collections is going to be very handy, so is probably my favourite of the four feature groups.

Read more at the C# Futures site.

Kirk