>I'm looking for the pros and cons of each communication options between a VFP front-end and a C# middle-end (SQL Server as a back-end).

I am using the first one you mentioned. In the migration of this product, when doing it step by step, I need a way to have the VFP application to benefit of one method I have in my .NET framework. So, I had to do a low scale version of that and make it COM interop. Once registered on the server, the VFP application was then able to communicate to that method.