No. You can't use .NET 4.0 with VS2008, and you can't use the C# 4.0 features either - it's exactly the same situation as trying to make VS2005 cope with C# 3.0. The editor, Intellisense etc just don't know about the new features.

一个重要的方面是，即使它看起来像像Visual Studio是只是使用外部 csc.exe 编译器，它不是 - 。进程内编译器与它附带的Visual Studio的版本绑定，所以它只知道当时可用的语言功能。

One important aspect is that even though it looks like Visual Studio is just using the external csc.exe compiler, it isn't - there's another "in-process" one. That in-process compiler is tied to the version of Visual Studio it comes with, so it only knows about the language features which were available at the time.