A application test for C# Reflection: fix source code

I have a basic WinForm Solution (MS VS2013, .Net framework 4.5) for test some methods included in a Dll, using Reflection. My goal is test the main application that it can run two methods of Dll (without referencing the dll in project), and later, run four methods (2 methods added) without stop the main application and run it again, using reflection. Reflections works fine (if I stop the main application, replacing the dll file and run the main application again, all works fine), but I can't replace the dll at runtime. The main application has a Timer control with an interval of 60 seconds. Every 60 seconds, a method is executed that checks if a DLL file is in a folder. If a DLL file exists in that folder, I want to use the new DLL in the main application (running) because the new DLL contains old methods (first DLL) and additional methods that the main application needs. However, I am getting an error that the file is in use. I have read several posts, questions, answers, MEF documentation, AppDomains related, but I have been unable to concatenate the information to be able to implement a solution. Actually, I thought a lot before post this question, but I confess that I prefer to spend a moment of shame, knowing that you can give me a hand. It would be for me a great help if you help me with source code corrections and specific documentation. Code attached (in plain txt): - Main application - Class in project, for some functions for support Main application - DLL source ([login to view URL]) Thanks.