Create An Application That Supports Addins?

I have 5 different applications that I have written for my business, they are all interfaces for a point of sale system. I TCP/IP interface handling part of the application is rock solid, problem is that occationally I will come across a small issue, such as a memory leak that I came across a few weeks ago. After I fixed it, I had to then fix said leak in 5 different projects as they all use the same interface base code.Now, if things were different, say a function that all my applications used I would just write it into a class in a dll and have all my applications use the function from that class. Problem is however, using a parent/child explanation unlike my example here where I have interface 1 - 5 as my parents using common.dll as a common child. I need to have InterfaceParent using 5 different children with different attributes.That being said, there are a few stipulations, depending on the customer I may only need to install interface 1 and 3, or 2 and 5, or 1,3 and 4.