C# application control flow determination

I have a third party开发者_开发百科 .Net application with literally hundreds of dlls and XML files. I need to add a single service to the application, and in the process need to find out which dlls are loaded, and which methods are invoked when a single button in the UI is pressed. The application and most of the dlls are in .Net. Any ideas on how to go about doing this? Is there any free software that I can use?

Have a look on NDepend. Its very helpfull to discover dependencies (and much more).