This is the official CODA specification in English from the Belgian Bankers’ and Stockbroking Firms’ Association (Febelfin).
You are unlikely to find a C# library to work with this - banks are notorious for having systems from the iron age. Maybe if you wanted a Cobol or Fortran implementation you'll find something ;)
The document discusses SEPA for local transactions, and apparently SWIFT for international. SWIFT is the de facto banking wire format for all international transactions. SWIFT has several modern implementations in most languages, though it's simple enough to implement yourself in any way you want.