为什么CoUninitialize在退出时会导致错误？

Question

我正在研究一个C++应用程序来读取Excel文件中的一些数据.我有它的工作,但我对一部分感到困惑.这是代码(简化为只读取第一个单元格).

//Mostly copied from http://www.codeproject.com/KB/wtl/WTLExcel.aspx

#import "c:\Program Files\Common Files\Microsoft Shared\OFFICE11\MSO.DLL"
#import "c:\Program Files\Common Files\Microsoft Shared\VBA\VBA6\VBE6EXT.OLB"
#import "C:\Program Files\Microsoft Office\Office11\excel.exe" rename ("DialogBox","ExcelDialogBox") rename("RGB","ExcelRGB") rename("CopyFile", "ExcelCopyFile") rename("ReplaceText", "ExcelReplaceText") exclude("IFont", "IPicture")

_variant_t varOption((long) DISP_E_PARAMNOTFOUND, VT_ERROR);

int _tmain(int argc, _TCHAR* argv[])
{
    DWORD dwCoInit = 0;
    CoInitializeEx(NULL, dwCoInit);
    Excel::_ApplicationPtr pExcel;    
    pExcel.CreateInstance(_T("Excel.Application"));
    Excel::_WorkbookPtr pBook;
    pBook = pExcel->Workbooks->Open("c:\\test.xls", varOption, varOption, varOption, varOption, varOption, varOption, varOption, varOption, varOption, varOption, varOption, varOption);
    Excel::_WorksheetPtr pSheet = pBook->Sheets->Item[1];
    Excel::RangePtr pRange = pSheet->GetRange(_bstr_t(_T("A1")));
    _variant_t vItem = pRange->Value2;
    printf(_bstr_t(vItem.bstrVal));    
    pBook->Close(VARIANT_FALSE);
    pExcel->Quit();
    //CoUninitialize();
    return 0;
}

我不得不评论对CoUninitialize的调用以使程序正常工作.取消注释CoUninitialize时,我在程序退出时在comip.h中的_Release函数中获得访问冲突.

这是来自comip.h的代码,它的价值.

void _Release() throw()
{
    if (m_pInterface != NULL) {
        m_pInterface->Release();
    }
}

我对COM编程不是很有经验,所以可能有一些我不知道的东西.

1. 为什么调用CoUninitialize会导致异常？

2. 不调用CoUninitialize有什么后果？

3. 我在这里做错了吗？

Answer 1

你遇到的问题是范围之一.简短的回答是将CoInit和CoUninit移动到Ptrs的外部范围.例如:

//Mostly copied from http://www.codeproject.com/KB/wtl/WTLExcel.aspx

#import "c:\Program Files\Common Files\Microsoft Shared\OFFICE11\MSO.DLL"
#import "c:\Program Files\Common Files\Microsoft Shared\VBA\VBA6\VBE6EXT.OLB"
#import "C:\Program Files\Microsoft Office\Office11\excel.exe" rename ("DialogBox","ExcelDialogBox") rename("RGB","ExcelRGB") rename("CopyFile", "ExcelCopyFile") rename("ReplaceText", "ExcelReplaceText") exclude("IFont", "IPicture")

_variant_t varOption((long) DISP_E_PARAMNOTFOUND, VT_ERROR);

int _tmain(int argc, _TCHAR* argv[])
{
    DWORD dwCoInit = 0;
    CoInitializeEx(NULL, dwCoInit);
    {
        Excel::_ApplicationPtr pExcel;    
        pExcel.CreateInstance(_T("Excel.Application"));
        Excel::_WorkbookPtr pBook;
        pBook = pExcel->Workbooks->Open("c:\\test.xls", varOption, varOption, varOption, varOption, varOption, varOption, varOption, varOption, varOption, varOption, varOption, varOption);
        Excel::_WorksheetPtr pSheet = pBook->Sheets->Item[1];
        Excel::RangePtr pRange = pSheet->GetRange(_bstr_t(_T("A1")));
        _variant_t vItem = pRange->Value2;
        printf(_bstr_t(vItem.bstrVal));    
        pBook->Close(VARIANT_FALSE);
        pExcel->Quit();
    }
    CoUninitialize();
    return 0;
}

更长的答案是在从main退出时调用Ptrs析构函数(调用Release).这是在CoUnit之后,它基本上关闭了你的应用程序和COM对象之间的通信通道.

不调用CoUnit有什么后果？对于短期进程中的COM服务器,确实没有任何负面后果.

Answer 2

一个优雅的解决方案是将 CoInitializeEx 和 CoUninitialize 放在它们自己的类中。请参阅这篇Raymond Chen 文章。