#include <usbfnioctl.h>  
#define   PSZ_REG_SERIAL_CLASS_CLIENT_DRIVER            _T("Serial_Class")
#define   PSZ_REG_MASS_STORAGE_CLASS_CLIENT_DRIVER      _T("Mass_Storage_Class")  
/**************************************************************************
 * function : get USB mode : activesync or mass storage
 * parameters   :  N/A
 * returns : TRUE : mass storage , FALSE :   activesync
 **************************************************************************/
BOOL   GetUSBMode()
{
 BOOL bRet = FALSE;
 HANDLE hHandle  =   CreateFile(L"UFN1:",FILE_WRITE_ATTRIBUTES|GENERIC_READ|GENERIC_WRITE,0,NULL,OPEN_EXISTING,0,NULL);
 if(hHandle   != INVALID_HANDLE_VALUE)
 {
  DWORD   dwBytes=0;
  UFN_CLIENT_INFO   ufnCInfo;
  DeviceIoControl(hHandle, IOCTL_UFN_GET_CURRENT_CLIENT,   NULL, 0, &ufnCInfo, sizeof(UFN_CLIENT_INFO), &dwBytes,   0);
  CloseHandle(hHandle);  
  if(_tcsncmp(ufnCInfo.szName, PSZ_REG_SERIAL_CLASS_CLIENT_DRIVER,   _tcslen(PSZ_REG_SERIAL_CLASS_CLIENT_DRIVER)))
  {
   bRet   = TRUE;
  }
 }
 return FALSE;
}  
/**************************************************************************
 *   function : set USB mode : activesync or mass storage
 * parameters   :   TRUE : mass storage , FALSE : activesync
 * returns :   success : TRUE , failed :   FALSE
 **************************************************************************/
BOOL   SetUSBMode(BOOL bIsUsbMode)
{
 BOOL bRet = FALSE;
 HANDLE   hHandle  =   CreateFile(L"UFN1:",FILE_WRITE_ATTRIBUTES|GENERIC_READ|GENERIC_WRITE,0,NULL,OPEN_EXISTING,0,NULL);
 if(hHandle   != INVALID_HANDLE_VALUE)
 {
  UFN_CLIENT_NAME   ufnClientName;
  if(bIsUsbMode)
  {
   _tcsncpy(ufnClientName.szName,PSZ_REG_MASS_STORAGE_CLASS_CLIENT_DRIVER,_tcslen(PSZ_REG_MASS_STORAGE_CLASS_CLIENT_DRIVER));
   ufnClientName.szName[_tcslen(PSZ_REG_MASS_STORAGE_CLASS_CLIENT_DRIVER)]   =   L'\0';
  }
  else
  {
   _tcsncpy(ufnClientName.szName,PSZ_REG_SERIAL_CLASS_CLIENT_DRIVER,_tcslen(PSZ_REG_SERIAL_CLASS_CLIENT_DRIVER));
   ufnClientName.szName[_tcslen(PSZ_REG_SERIAL_CLASS_CLIENT_DRIVER)]   = L'\0';
  }  
  DWORD dwBytes;
  bRet = DeviceIoControl(hHandle,   IOCTL_UFN_CHANGE_CURRENT_CLIENT, ufnClientName.szName,sizeof(UFN_CLIENT_NAME),   NULL, 0,   &dwBytes,0);
  CloseHandle(hHandle);
  
  return   bRet;
 }
 return bRet;
}

要使重启后继续有效还需要修改以下注册表:
[HKEY_LOCAL_MACHINE\Drivers\USB\FunctionDrivers]
"DefaultClientDriver"="Serial_Class"
;改为Mass_Storage_Class为存储