为什么当我添加对MEF插件项目的引用时，会得到一个警告图标?

I wish to test the core class of a plugin by directly referencing the plugin project and instantiating the plugin class. When I create a test Console App project and add a project reference to the plugin project, I get a warning icon (yellow triangle with exclamation mark) next to the reference in the References list. I wish to test the core class of a plugin by di