C Sharp 4.0

C# 4.0 is a version of the C# programming language that was released on April 11, 2010. Microsoft released the 4.0 runtime and development environment Visual Studio 2010. The major focus of C# 4.0 is interoperability with partially or fully dynamically typed languages and frameworks, such as the Dynamic Language Runtime and COM.

Features

The following new features were added in C# 4.0.

Dynamic member lookup

A new pseudo-type dynamic is introduced into the C# type system. It is treated as System.Object, but in addition, any member access (method call, field, property, or indexer access, or a delegate invocation) or application of an operator on a value of such type is permitted without any type checking, and its resolution is postponed until run-time. This is known as duck typing. For example:

// Returns the value of Length property or field of any object
int GetLength(dynamic obj)

GetLength("Hello, world"); // a string has a Length property,
GetLength(new int[] ); // and so does an array,
GetLength(42); // but not an integer - an exception will be thrown in GetLength method at run-time

Dynamic method calls are triggered by a value of type dynamic as any implicit or explicit parameter (and not just a receiver). For example:

void Print(dynamic obj)

Print(123); // ends up calling WriteLine(int)
Print("abc"); // ends up calling WriteLine(string)

Dynamic lookup is performed using three distinct mechanisms: COM IDispatch for COM objects, IDynamicMetaObjectProvider DLR interface for objects implementing that interface, and reflection for all other objects. Any C# class can therefore intercept dynamic calls on its instances by implementing IDynamicMetaObjectProvider.

In case of dynamic method and indexer calls, overload resolution happens at run-time according to the actual types of the values passed as arguments, but otherwise according to the usual C# overloading resolution rules. Furthermore, in cases where the receiver in a dynamic call is not itself dynamic, run-time overload resolution will only consider the methods that are exposed on the declared compile-time type of the receiver. For example:

class Base

class Derived : Base

dynamic x = 123;
Base b = new Derived();
b.Foo(x); // picks Base.Foo(double) because b is of type Base, and Derived.Foo(int) is not exposed
dynamic b1 = b;
b1.Foo(x); // picks Derived.Foo(int)

Any value returned from a dynamic member access is itself of type dynamic. Values of type dynamic are implicitly convertible both from and to any other type. In the code sample above this permits GetLength function to treat the value returned by a call to Length as an integer without any explicit cast. At run time the actual value will be converted to the requested type.

Covariant and contravariant generic type parameters

Generic interfaces and delegates can have their type parameters marked as covariant or contravariant using keywords out and in respectively. These declarations are then respected for type conversions, both implicit and explicit, and both compile time and run time. For example, the existing interface IEnumerable has been redefined as follows:

interface IEnumerable

Therefore, any class that implements IEnumerable for some class Derived is also considered to be compatible with IEnumerable for all classes and interfaces Base that Derived extends, directly or indirectly. In practice, it makes it possible to write code such as:

void PrintAll(IEnumerable