在Visual Studio中，是否可以添加加载项程序集作为解决方案中另一个程序集的引用？

I am using Visual Studio 2010. I have multiple C# projects in my solution. One of the projects is an Excel Add-In. I want to be able to write unit tests on some of the code that I have in the Excel Add-In project. To do this, I have created another project in the solution. I want this test project to be able to reference the Add-In project. However, the Add-In project does not show up on the Projects tab of the Add Reference dialog. The only way that I have been able to do anything is by adding the Add-In dll file as a reference in the test project. This is not ideal, because a new dll would have to be generated whenever changes are made in the Add-In project. Is this a limitation of Excel Add-In projects? Or do I need to set the Excel Add-In project up in a particular way to make it visible to the test project? I am using Visual Studio 2010. I have multiple