在Visual Studio中，您可以选择一个目标框架".
选择.NET Framework 3.5，然后Visual Studio将您的代码编译到.NET 3.5及更高版本的应用程序中.
要找到此选项，请转到属性"->应用程序".

In Visual Studio, you can choose a "Target framework".
Select the .NET Framework 3.5 and then Visual Studio compiles your code to a application for .NET 3.5 and higher.

To find this option, go to Properties->Application.