New features in C# 4.0

The major theme for C# 4.0 is dynamic programming. Increasingly, objects are “dynamic” in the sense that their structure and behavior is not captured by a static type, or at least not one that the compiler knows about when compiling your program.



Some examples include
a.    objects from dynamic programming languages, such as Python or Ruby
b.    COM objects accessed through IDispatch
c.    ordinary .NET types accessed through reflection
d.    objects with changing structure, such as HTML DOM objects

While C# remains a statically typed language, we aim to vastly improve the interaction with such objects.
A secondary theme is co-evolution with Visual Basic. Going forward we will aim to maintain the individual character of each language, but at the same time important new features should be introduced in both languages at the same time. They should be differentiated more by style and feel than by feature set.

The new features in C# 4.0 fall into four groups:

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.