Hi Mykyta,

Can you make sure that the ZOSAPI.dll and ZOSAPI_Interfaces.dll that you added in the Reference section of the Solution Explorer is set to Copy Local False (the ZOSAPI_NetHelper.dll should be Copy Local True):

The ZOSAPI_NetHelper.dll is a standalone class (the Connection class) which searches through a series of directories trying to locate OpticStudio and this is why this DLL needs to be in the same directory as your ZOS-API executable.  The ZOSAPI.dll and ZOSAPI_Interfaces.dll have relative paths for the rest of OpticStudio (all the internal libraries, 3rd party libraries, and the OpticStudio.exe itself).  If you have Copy Local True for these DLLs, then your Standalone Application will copy these 2 DLLs to the same directory as your executable and when these DLLs search for the rest of OpticStudio, they return a null reference.  You need to include the ZOSAPI.dll and ZOSAPI_Interfaces.dll so Visual Studio can provide intellisense and can compile, but the Connection class will dynamically load these DLLs from their local directory via reflection when you execute your application.