Well, how about providing some details? Such as what C# functions? Is this a private assembly? Is it something from the standard .NET library? What version of LabVIEW? Are you running 32-bit or 64-bit OS? 32-bit or 64-bit LabVIEW? Is the assembly 32-bit or 64-bit? That error can happen when you try to call a 32-bit library in a 64-bit environment. If that is the case do you have a 64-bit version?