这一块code编译一个框架3.5的项目在VS 2010中确定（我三重检查了）

This piece of code compiles OK in VS 2010 in a framework 3.5 project (I triple checked that)

    public LoggingClient(string uri = "net.msmq://localhost/logging"){...}

为什么呢？我什么也看不到在C＃4规格（doc版本），第21.1节，是说，这应该是向后兼容。它怎么说，我没有得到任何编译错误？这是否会在某些情况下静默失败？

Why? I see nothing in the C# 4 spec (doc version), section 21.1, that says this should be backwardly compatible. How is it that I get no compilation error? Will this fail silently in some circumstances?

推荐答案

项目+属性，生成选项卡，向下滚动，高级。您可以更改语言版本为C＃3.0，如果你preFER保持源$ C ​​$ C兼容性。

Project + Properties, Build tab, scroll down, Advanced. You can change the Language Version to "C# 3.0" if you prefer to maintain source code compatibility.

但是，是的，你使用的是C＃4.0编译器在VS2010，无论你使用的目标.NET版本。编译器，IL的输出，并没有改变在.NET 4.0中。不，你不能使用的动态的，它需要一个.NET 4.0只支持组件（Microsoft.CSharp.dll）

But yes, you are using the C# 4.0 compiler in VS2010, regardless of the target .NET version you use. The output of the compiler, IL, hasn't changed in .NET 4.0. No, you can't use dynamic, it requires a .NET 4.0 only support assembly (Microsoft.CSharp.dll)