C / C++ / MFC

typedef unsigned char UN_Char;
typedef enum { Valid = 0, Invalid = 1, Unknown = -999 } AppResponseCode;

static AppResponseCode SomeFunction(UN_Char ** vUC1PtrPtr,size_t * vSize1Ptr, UN_Char * vUC2Ptr,size_t vSize2, FILE *  inputFile, char * vcharPtr)

A Reference to '.....\SomeApi.DLL' could not be added. Please make sure that the file is accessible, and that it is valid assembly or COM Component

[DllImport("SomeAPI.dll", CharSet = CharSet.Unicode)]
public static extern AppResponseCode SomeFunction( ????? ) ;