VS2010默认只安装.net framework 4.0,如果要用2.0,3.0,3.5等框架,则需要安装.net framework 3.5 sp1,请看这篇博客:Framework .NET 3.5 Sp1 required for targeting frameworks 2.0, 3.0 or 3.5 (multi-targeting) using Visual Studio 2010Visual Studio 2010 选择.NET 2.0/3.0/3.5框架不能编译的解决办法