Build a Plugin Mechanism in c#

We need to develop a plugin mechanism for our application, in order to modularize the source code. The mechanism has to provide the following functionalities: 1. It should be able to load external libraries (DLL) into the main application in both desktop and web applications. 2. It should offer the best security technics for loading external libraries 3. It has to be able to load the external libraries from a remote location, at runtime and when needed. 4. It should provide a general mechanism for external libraries registering and make the loaded functionalities globally available in the application context. 5. It should be compliant with best practices in regards to loading and execute external code in both environments desktop and web. (Sandboxed and cross-domain policies files) 6. It should support multithreading, asynchronism, and parallelism for loaded functionalities 7. It should provide an error management system to be easily integrated with the main error management system provided by the framework. 8. It should provide a generic messaging system between both main application and external plugins and between any other plugins. 9. It should be able to load and made them discoverable both types of external libraries, the ones who deliver logic and the ones who deliver resources. 10. It should reference NO external libraries and it should be built as a library which can be referenced by the main application. 11. It should be model/entity data changing and data changed aware. The mechanism will be built in c-sharp using .net Framework 4.5.2