您不能添加对.NET Core项目的引用

You cannot add a reference to a .NET Core project to a .NET Framework project.

要在.NET Core和.NET Framework应用程序/库之间共享代码，共享项目必须以.NET Standard为目标。如果创建.NET Standard 2.0项目，则可以在.NET Framework 4.6.1+项目和.NET Core 2.0+项目中使用。

To share code between .NET Core and .NET Framework apps / libraries, the shared project must target .NET Standard. If you create a .NET Standard 2.0 project, it can be used in .NET Framework 4.6.1+ projects and .NET Core 2.0+ projects.