SafeArrayTypeMismatchException when passing c# struct to unmanaged C++ DLL -

hi i'm new c# , c++ programming , trying use unmanaged c++ dll in c# project (.net 3.5). i'm stuck on error:

system.runtime.interopservices.safearraytypemismatchexception: specified array not of expected type.

here header file dll

#ifdef functionlib_exports #define functionlib_api __declspec(dllexport) #else #define functionlib_api __declspec(dllimport) #endif typedef struct _functionopt  {     char                username[32];     char                password[32];     char                serverip[128];     int                 serverport;     int                 index;               char                targetsubchannel;     long                timeout;     char                filepath[256];     bool                isfirst;             dword               synctime;                    char                *poutbuf;                    int                 outbufsize;     unsigned short      outimgresw;     unsigned short      outimgresh;  }stfunctionopt, *pstfunctionopt;  functionlib_api int functionlib_snapshot( pstfunctionopt pstfunctionopt );  

i don't have access c++ code can change following c# code. relevant code follows:

    public unsafe struct pstfunctionopt                 {         public char[] username;         public char[] password;         public char[] serverip;         public int serverport;         public int index;                               public char targetsubchannel;                       public long timeout;                       public char[] filepath;         public bool isfirst;                        public uint synctime;                                public char *poutbuf;   // example c++ usage: mystruct.poutbuf = (char*)malloc(1920 * 1080 * 3);         public int outbufsize;         public ushort outimgresw;         public ushort outimgresh;     }      // need use dumpbin entrypoint name due c++ mangling     [dllimport(@"c:\location\to\project\bin\functionliblib.dll", entrypoint = "?functionlib_snapshot@@yahpau_functionopt@@@z")]     public static extern int functionlib_snapshot(pstfunctionopt pstfunctionopt);      public unsafe int functionlib_run()     {         pstfunctionopt stfunctionopt = new pstfunctionopt();          stfunctionopt.username = ("uname").tochararray();         stfunctionopt.password = ("pword").tochararray();         stfunctionopt.serverip = ("192.168.1.1").tochararray();         stfunctionopt.serverport = 80;         stfunctionopt.index = 255;         stfunctionopt.timeout = 15000;         stfunctionopt.filepath = ("c:\\temp\\test.jpg").tochararray();         stfunctionopt.isfirst = true;         stfunctionopt.synctime = 0;     //stfunctionopt.poutbuf = new char*[10000];     // not sure how yet         stfunctionopt.outbufsize = 10000;          // result dll          return functionlib_snapshot(stfunctionopt);     } 

how pass struct unmanaged dll? error seems simple haven't been able narrow down problem. appreciated!

several problems:

    public char[] username; 

that needs marshaled embedded string:

    [marshalas(unmanagedtype.byvaltstr, sizeconst = 32)]     public string username; 

repeat other fields declared this.

    public char *poutbuf;   // example c++ usage: mystruct.poutbuf = (char*)malloc(1920 * 1080 * 3); 

it not clear whether c++ function allocates buffer or client code supposed that. if former have big problem, cannot call free() function release buffer again. hope c# code supposed allocate it, not unlikely. in case is:

    public byte[] poutbuf;     ...     stfunctionopt.poutbuf = new byte[10000];     stfunctionopt.outbufsize = stfunctionopt.poutbuf.length; 

the pinvoke declaration wrong:

[dllimport(@"c:\location\to\project\bin\functionliblib.dll", entrypoint = "?functionlib_snapshot@@yahpau_functionopt@@@z")] public static extern int functionlib_snapshot(pstfunctionopt pstfunctionopt); 

the argument ref pstfunctionopt pstfunctionopt. drop p structure name, not pointer type. avoid hard-coding path dll, isn't going work on user's machine. make sure have copy of dll in build output directory.