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Reading Strings into the OLE DB Provider

 

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The latest version of this topic can be found at Reading Strings into the OLE DB Provider.

The RMyProviderRowset::Execute function opens a file and reads strings. The consumer passes the file name to the provider by calling ICommandText::SetCommandText. The provider receives the file name and stores it in the member variable m_szCommandText. Execute reads the file name from m_szCommandText. If the file name is invalid or the file is unavailable, Execute returns an error. Otherwise, it opens the file and calls fgets to retrieve the strings. For each set of strings it reads, Execute creates an instance of the user record (CAgentMan) and places it into an array.

If the file cannot be opened, Execute must return DB_E_NOTABLE. If it returns E_FAIL instead, the provider will not work with many consumers and will not pass the OLE DB conformance tests.

Description

The edited Execute function looks like this:

Code

/////////////////////////////////////////////////////////////////////////  
// MyProviderRS.h  
class RMyProviderRowset : public CRowsetImpl< RMyProviderRowset, CAgentMan, CRMyProviderCommand>  
{  
public:  
    HRESULT Execute(DBPARAMS * pParams, LONG* pcRowsAffected)  
    {  
        enum {  
            sizeOfBuffer = 256,  
            sizeOfFile = MAX_PATH  
        };  
        USES_CONVERSION;  
        FILE* pFile = NULL;  
        TCHAR szString[sizeOfBuffer];  
        TCHAR szFile[sizeOfFile];  
        size_t nLength;        errcodeerr;  
  
        ObjectLock lock(this);  
  
        // From a filename, passed in as a command text, scan the file  
        // placing data in the data array.  
        if (!m_szCommandText)  
        {  
            ATLTRACE("No filename specified");  
            return E_FAIL;  
        }  
  
        // Open the file  
        _tcscpy_s(szFile, sizeOfFile, m_szCommandText);  
        if (szFile[0] == _T('\0') ||   
            ((err = fopen_s(&pFile, &szFile[0], "r")) == 0))  
        {  
            ATLTRACE("Could not open file");  
            return DB_E_NOTABLE;  
        }  
  
        // Scan and parse the file.  
        // The file should contain two strings per record  
        LONG cFiles = 0;  
        while (fgets(szString, sizeOfBuffer, pFile) != NULL)  
        {  
            nLength = strnlen(szString, sizeOfBuffer);  
            szString[nLength-1] = '\0';   // Strip off trailing CR/LF  
            CAgentMan am;  
            _tcscpy_s(am.szCommand, am.sizeOfCommand, szString);  
            _tcscpy_s(am.szCommand2, am.sizeOfCommand2, szString);  
  
            if (fgets(szString, sizeOfBuffer, pFile) != NULL)  
            {  
                nLength = strnlen(szString, sizeOfBuffer);  
                szString[nLength-1] = '\0'; // Strip off trailing CR/LF  
                _tcscpy_s(am.szText, am.sizeOfText, szString);  
                _tcscpy_s(am.szText2, am.sizeOfText2, szString);  
            }  
  
            am.dwBookmark = ++cFiles;  
            if (!m_rgRowData.Add(am))  
            {  
                ATLTRACE("Couldn't add data to array");  
                fclose(pFile);  
                return E_FAIL;  
            }  
        }  
  
        if (pcRowsAffected != NULL)  
            *pcRowsAffected = cFiles;  
        return S_OK;  
    }  
}  

Implementing the Simple Read-Only Provider

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