C#4.0，.NET Framework 4.0， Visual Studio 2010
The newly introduced syntax of C# 4.0 is based on .Net Framework 4.0. Mainly introduced syntax: dynamic typing, named parameters, optional parameters, optimized Com calls
The most important of these are dynamic types
Dynamic binding: Dynamic binding, that is, dynamic type, determines its type at runtime. Use the keyword dynamic to define
Named and optional arguments: Named and optional arguments
Generic co- and contravariance: Covariance and contravariance of generics, increase generics and use in/out to correspond to covariance and contravariance.
Embedded interop types ("NoPIA"): Enable embedded type information to increase the neutrality of referencing COM component programs. Eliminate unnecessary parameters when calling the Com interface.