Well, it depends on what the dll is for. i.e. You can make a dll that provides the functionality of a binary tree (i.e. class BinaryTree). Then using that dll will allow you to have the class BinaryTree in any project you want. So basically you implement the class BinaryTree in the class library project while you use another project to test it properly (yes, 2 projects in the solution). In this case in the testing project you need to add a reference to the dll project. Just right click on the References folder in your Solution Explorer, choose "Add Reference", the open the tab Projects and select your class library.
There's just one problem. Creating a dll that is really well portable into any project isn't simple. You have to create a good interface or else there's not really a point in making a dll.