Getting Access Violation when call vc++ dll from Vb.net

Question

The error is " AccessViolationException: Attempted to read or write protected memory "

vc++ DLL

#ifndef IZZIXFELAPI32_H
#define IZZIXFELAPI32_H

#ifdef IZZIXFELAPI32_EXPORTS
#define IZZIXFELAPI32_API __declspec(dllexport)
#else
#define IZZIXFELAPI32_API __declspec(dllimport)
#endif

#define MAX_FEATUREVECT_LEN 480
#define HIGH_LEVEL          2
#define MEDIUM_LEVEL        1
#define LOW_LEVEL           0

// Error code of FingerAPI 
#define     FPAPIERR_NO                      0  
#define     FPAPIERR_OK                      1
#define     FPAPIERR_GENERAL_ERROR          -1
#define     FPAPIERR_CAN_NOT_OPEN_DEVICE    -2

#define     FPAPIERR_MATCH_FAILED           -101
#define     FPAPIERR_CAN_NOT_ALLOC_MEMORY   -201
#define     FPAPIERR_VECT_FAILED            -301  
#define     FPAPIERR_INVALID_IMAGESIZE      -401
#define     FPAPIERR_FAKER_FINGERPRINT      -501

#define     FPAPIERR_LEFT_FINGERPRINT       -601
#define     FPAPIERR_RIGHT_FINGERPRINT      -602
#define     FPAPIERR_UP_FINGERPRINT         -603
#define     FPAPIERR_DOWN_FINGERPRINT       -604
#define     FPAPIERR_SMALL_FINGERPRINT      -605

#define     FPAPIERR_TOO_WET                -701
#define     FPAPIERR_TOO_DRY                -702


typedef struct
{
    BYTE Data[MAX_FEATUREVECT_LEN];
} MINUTIAVECT, *LPMINUTIAVECT;


#ifdef __cplusplus
#define EXTWRN_C            extern "C"
#else
#define EXTWRN_C
#endif

#ifdef __cplusplus
extern "C" {
#endif


namespace IzzixFELAPI32
{

void __stdcall DESEncode0(UCHAR *src,UCHAR *dec,UCHAR *key,int num);

void __stdcall DESEncode1(UCHAR *src,UCHAR *dec,UCHAR *key);

int __stdcall DESDecode(UCHAR *src,UCHAR *dec,UCHAR *key);

int __stdcall GetFinger(INT DeviceNumber,BYTE* pRawImage,LPMINUTIAVECT pFeature);

int __stdcall MatchFingerOneToOne(LPMINUTIAVECT pFeatureVect1,
                               LPMINUTIAVECT pFeatureVect2,
                               int securitylevel
                              );

int __stdcall MatchFingerOneToN(
                    LPMINUTIAVECT pFeatureVect,
                    LPMINUTIAVECT pDataBaseVects,
                    int nDataBaseSize,
                    int securitylevel,
                    int* pDataBaseIDs,
                    int* pAlwaysNULL
                    );

int __stdcall CollectFeature(LPMINUTIAVECT pFeatureVect1,
                          LPMINUTIAVECT pFeatureVect2,
                          LPMINUTIAVECT pFeatureVect3,
                          LPMINUTIAVECT pCollectVect
                          );


int __stdcall DisplayImage(HWND hWnd,int x1,int y1,int x2,int y2,
                        BYTE* pImage,int nWidth,int nHeight);

int __stdcall IsAvailableDevice(INT DeviceNumber);

int __stdcall GetSortedIndex(LPMINUTIAVECT pFeatureVect,
                    LPMINUTIAVECT pDataBaseVects,
                    int nDataBaseSize,
                    int* pIndex
                   );

int __stdcall GetFingerAPIVersion(UCHAR *Version);

int __stdcall GetSerial(INT DeviceNumber,UCHAR *Serial);


int __stdcall ConvertToISO(LPMINUTIAVECT pFeature,int fPos,unsigned char* Template);

int __stdcall ConvertFromISO(unsigned char* Template,LPMINUTIAVECT pFeature);

int __stdcall GetTouchStatus(INT DeviceNumber, BOOL &bTouch);

int __stdcall GetDevInfos(INT DeviceNumber, int *nProductType, int *nSensorType);

}


#ifdef __cplusplus
}
#endif

#endif

vb.net code is

<DllImport("IzzixFELAPI32.dll")> _
Private Shared Function GetSerial(ByVal DeviceNumber As Long, ByVal serial As     StringBuilder) As Int32
End Function

...
Dim sb As New StringBuilder(550)
Call GetSerial(0, sb)
Dim st As String = sb.ToString