Sharing an Interface (What not to do)

Have you ever had a class library that uses an Interface?  How about two or more libraries that need to use the same Interface?  For instance, ClassLibrary1 and ClassLibrary2 need to implement ISomeInterface below:

Instead of copying ISomeInterface.vb to both ClassLibrary projects or adding a project reference, you can add a link to ISomeInterface.vb and have access to ISomeInterface:

Now you’ll notice that ISomeInterface.vb is part of ClassLibrary1 as a linked file:

Now we can implement ISomeInterface in ClassLibrary1:

The benefit of adding the Interface file as a link is that you can maintain the Interface in one location.  You don’t have to worry about having to copy new versions if you decide to change your contract.