您的项目至少需要面向 .NET Framework 4.您可以在项目属性下更改此设置.

You need to target at least .NET Framework 4 in your project. You can change this under project properties.

此外，更改目标不会为库添加引用，因此您必须在更改目标框架后手动执行此操作.

Also, changing the target will not add the reference for the library, so you have to do this manually after changing the target framework.