仅在Windows XP上诊断失败的P/Invoke调用需要帮助

我定义了以下p/Invoke：

[DllImport("helper.dll", CallingConvention=CallingConvention.Cdecl, 
    CharSet=CharSet.Ansi, EntryPoint="F_GetValue")]
private static extern Int32 _F_GetValue(String Formula, ref DATA_STRUCT Data,
    ref DATA_KEY DefaultKeyBuf, ref Double Result);

此调用在Windows Vista及更高版本上成功，但在Windows XP上失败，出现以下内存异常：

A first chance exception of type 'System.AccessViolationException' occurred 
in helper.dll

我尝试将前两个"ref"修饰符更改为[In，Out]，但这并没有解决问题。

DATA_STRUCT和DATA_KEY都是实例化和预填充的结构。

以下是我正在调用的C++方法定义：

int F_GetValue(const char* pFormula, DATA_STRUCT* pData, 
    DATA_KEY* pDefaultKeyBuf, double* freturn)

我不是p/invoke大师，所以不要想当然。这个定义方式有什么明显的错误吗？是否还有更多封送处理要进行(手动(？我觉得我可能错过了一些显而易见的东西。

EDIT：根据要求，这里分别是.NET中的结构定义、C++F_GetValue((方法和C++结构定义：

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi, Pack = 1)]
public struct DATA_STRUCT
{
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 260)]
    public String DataDir;
    [MarshalAs(UnmanagedType.U2)]
    public UInt16 LTType;
    [MarshalAs(UnmanagedType.U2)]
    public UInt16 FOMType;
    [MarshalAs(UnmanagedType.U2)]
    public UInt16 ResultType;
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 136)] // ( POS_BLOCK_SIZE + sizeof(int) + 4 )
    public Byte[] posBlock;
    public DATA_REC dataBuf;
    [MarshalAs(UnmanagedType.U2)]
    public UInt16 dataLen;
    [MarshalAs(UnmanagedType.U2)]
    public UInt16 keyNum;
    public DATA_KEY keyBuf;
    [MarshalAs(UnmanagedType.U2)]
    public UInt16 TNTC;
    [MarshalAs(UnmanagedType.I2)]
    public Int16 status;
}
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi, Pack = 1)]
public struct DATA_KEY
{
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 17)]
    public String LocName;
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 17)]
    public String ParName;
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 13)]
    public String DateTime;
}
int F_GetValue(const char* pFormula, DATA_STRUCT* pData, DATA_KEY* pDefaultKeyBuf, double* freturn)
{
    if (pFormula[0] == 0) // return quickly if nothing to do
    {
        *freturn = blank;
        pData->ResultType = sbit(DATA_BLANK);
        pData->status = B_NO_ERROR;
        return 0;
    }
    if ((_strnicmp(pFormula, "DOTNET_", 7) == 0) || (_strnicmp(pFormula, "(DOTNET_", 7) == 0)) // switch to/from dotnet
    {
        dotnetCalcs = (_strnicmp(pFormula + 7 + ((pFormula[0] == '(') ? 1 : 0), "ON", 2) == 0);
        *freturn = dotnetCalcs ? 1 : 0;
        pData->ResultType = sbit(DATA);
        pData->status = B_NO_ERROR;
        return strlen(pFormula);
    }
    BOOL bComingFromDotNet = (pData->dataLen == 65535);
    if (dotnetCalcs && (!bComingFromDotNet))
    {
        return F_GetValue2(pFormula, pData, pDefaultKeyBuf, freturn);
    }
    if (pSharedMem->bClient && ! bServer)
    {
        if (FromServer(ACTION_OPEN,NULL) == B_NO_ERROR)
        {
            ((CS_FORMULA *)pSharedMem->ClientServer)->nRecords = 1;
            strcpy(((CS_FORMULA *)pSharedMem->ClientServer)->DataDir,pData->DataDir);
            memcpy(&((CS_FORMULA *)pSharedMem->ClientServer)->Formula[0].DefaultKeyBuf,pDefaultKeyBuf,sizeof(DATA_KEY));
            strcpy(((CS_FORMULA *)pSharedMem->ClientServer)->Formula[0].Formula,pFormula);
            ((CS_FORMULA *)pSharedMem->ClientServer)->iType = CSTYPE_FORMULA;
            ((CS_FORMULA *)pSharedMem->ClientServer)->iAction = ACTION_READ;
            if (FromServer(ACTION_READ,NULL) == B_NO_ERROR)
            {
                *freturn = ((CS_FORMULA_RESULT *)pSharedMem->ClientServer)->Data[0].Data;
                pData->ResultType = ((CS_FORMULA_RESULT *)pSharedMem->ClientServer)->Data[0].ResultType;
                pData->status = ((CS_FORMULA_RESULT *)pSharedMem->ClientServer)->Data[0].status;
                FromServer(ACTION_CLOSE,NULL);
                return strlen(pFormula);
            }
            FromServer(ACTION_CLOSE,NULL);
        }
        *freturn = blank;
        return 0;
    }
    else
    {
        BOOL bOpenTemporaryData = bComingFromDotNet || (pData->dataLen == 0);
        if (bOpenTemporaryData)
        {
            DataOpenAndInitialize(pData,NULL);
        }
        int iReturn = F_DoGetValue(pFormula,pData,pDefaultKeyBuf,freturn);
        if (bOpenTemporaryData)
            DataExec(B_CLOSE,pData);
        return iReturn;
    }
}
typedef struct
{
    char DataDir[MAX_PATH];
    unsigned short LTType; 
    unsigned short FOMType; 
    unsigned short ResultType; 
    BTI_BYTE posBlock[POS_BLOCK_SIZE_];
    DATA_REC dataBuf;
    BTI_WORD dataLen;
    BTI_WORD keyNum;
    DATA_KEY keyBuf;
    unsigned short TNTC;
    BTI_SINT status;
} DATA_STRUCT;
typedef struct
{
char    LocName[LP_SIZE];
char    ParName[LP_SIZE];
char    DateTime[13];
} DATA_KEY;

为了完整起见，我还包含了这个方法F_GetValue2((，它在F_GetValue((中调用。虽然这看起来可能会直接返回到托管代码中，但它不会。这个方法存在的目的不同，我不能向你保证，在我的XP遇到问题的情况下，它不会被调用，因为它要求(dotnetCalcs&&(！bComingFromDotNet((为真，但事实并非如此。

还有一件事，其中调用的另一个方法是F_DoGetValue((，然后将参数集传递给它。这个方法很大，所以我不在这里发布它。但是，只要说它解析Formula，并使用它所学到的知识来调用更多的方法，这些方法使用解析的字符串从数据库中提取数据，将Double成员fReturn返回到链上，直到它最终通过封送处理返回到C#代码。

   [StructLayout(..., Pack = 1)]

__declspec( thread ) BOOL dotnetCalcs = FALSE;

if (dotnetCalcs && (!bComingFromDotNet))