my goal is to create a programm to exchange some to data (convert grf files) with another applicaton (both applications uses Visual Basic for Applications for automation) the best way we did this in the past was to have an external software c# which access both systems to the vba interop files. which interopt dll do i need to include for the GRF - Files and how is the best way to initialize the application ? my first guess was to use - Interop.FixApp my guess was to start with something like this namespace ConsoleApplication1 { class Program { static void Main(string[] args) { FixApp.CFixApp app = new FixApp.CFixAppClass(); } } } but i got an error: 800401f9 ... (i have seen other applicaitons which are using the same approach ... but i don't have any details ... so in principal it should work)