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: 
File > Open > Project/Solution > Add to Solution (A little checkbox in the file dialog) then click the .sln you want
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.