Question: I need to use all the classes in one project in another. I tried adding the references, clicked on the project tab but I can't see the `.cs` or `.sln` files or any other files, just the exe in the debug folder and the `.vshost` file and the manifest file.

What file do I need to reference in the project?

Answer: 
Right mouse click on the project that needs a reference to another project --> Add Reference --> Click the checkbox(es) next to each project you wish to refer.

[![Checkboxes and projects to refer to](https://i.stack.imgur.com/vzMDO.png)](https://i.stack.imgur.com/vzMDO.png)

Once you have added your reference you will need to include the namespace "using NamespaceOfSecondProject" in the class that will use the referred project.
You need to elaborate you question - Well why would you want to have redundant classes in two projects better build a class library and reference the dll in both the projects. If at all you need to include the classes from one project to another you simply need to copy the .cs file to other project and then select project and select add existing item then select the .cs file which you wanted to add. Hope this will help.