MDAC 2.5 SDK - ADOX

Attributes Property Example (VC++)

See Also

This example demonstrates the Attributes property of a Column. Setting it to adColNullable allows the user to set the value of a Recordset Field to an empty string. In this situation, the user can distinguish between a record where data is not known and a record where the data does not apply.

#import "c:\Program Files\Common Files\system\ado\msado15.dll"
#import "c:\Program Files\Common Files\system\ado\msadox.dll" \
    no_namespace

#include "iostream.h"
#include "stdio.h"
#include "conio.h"
#include "AttributesX.h"

//Function declarations
inline void TESTHR(HRESULT x) {if FAILED(x) _com_issue_error(x);};
void AttributesX(void);

//////////////////////////////////////////////////////////
//                                                      //
//         Main Function                                //
//                                                      //
//////////////////////////////////////////////////////////
void main()
{
    if(FAILED(::CoInitialize(NULL)))
        return;

    AttributesX();

    ::CoUninitialize();
}

//////////////////////////////////////////////////////////
//                                                      //
//        AttributesX Function                          //
//                                                      //
//////////////////////////////////////////////////////////
void AttributesX(void)
{
    HRESULT hr = S_OK;

    // Define ADOX object pointers.
    // Initialize pointers on define.
    // These are in the ADOX::  namespace.
    _CatalogPtr m_pCatalog  = NULL;
    _ColumnPtr m_pColumn  = NULL;
    _TablePtr m_pTable  = NULL;

    // Define ADODB object pointers
    ADODB::_ConnectionPtr m_pCnn         = NULL;
    ADODB::_RecordsetPtr m_pRstEmployees  = NULL;

    IADORecordBinding *picRs  = NULL;   // Interface Pointer Declared
    CEmployeeRs emprs;            // C++ Class Object

    // Define string variables.
    _bstr_t strcnn("Provider=Microsoft.Jet.OLEDB.4.0;"
        "Data Source= c:\\Program Files\\Microsoft Office\\"
        "Office\\Samples\\Northwind.mdb;");

    try
    {
        // Connect the catalog.
        TESTHR(hr = m_pCnn.CreateInstance(__uuidof (ADODB::Connection)));
        TESTHR(hr = m_pCatalog.CreateInstance(__uuidof (Catalog)));
        TESTHR(hr = m_pColumn.CreateInstance(__uuidof(Column)));
        TESTHR(hr = m_pRstEmployees.CreateInstance(
            __uuidof(ADODB::Recordset)));
        m_pCnn->Open(strcnn,"","",NULL);

        m_pCatalog->PutActiveConnection(
            _variant_t((IDispatch *) m_pCnn));
        m_pTable= m_pCatalog->Tables->GetItem("Employees");

        // Create a new Field object and append it to the Fields
        // collection of the Employees table.
        m_pColumn->Name = "FaxPhone";
        m_pColumn->Type = adVarWChar;
        m_pColumn->DefinedSize = 24;
        m_pColumn->Attributes = adColNullable;

        m_pCatalog->Tables->GetItem("Employees")->Columns->
            Append("FaxPhone",adVarWChar,24);

        // Open the Employees table for updating as a Recordset.
        m_pRstEmployees->Open("Employees",
            _variant_t((IDispatch *) m_pCnn),
            ADODB::adOpenKeyset,ADODB::adLockOptimistic,
            ADODB::adCmdTable);

        // Get user input.
        printf("Enter fax number for : %s %s\n",(LPSTR) (_bstr_t) 
            m_pRstEmployees->Fields->GetItem("LastName")->Value,
           (LPSTR) (_bstr_t) m_pRstEmployees->Fields->
            GetItem("FirstName")->Value);
        printf("[? - unknown, X - has no fax] : \n");
        char strInput[10];
        gets(strInput);
        char* strTemp = strtok(strInput," \t");
        _variant_t vNull;
        vNull.vt = VT_BSTR;
        vNull.bstrVal = NULL;
        if(strTemp!=NULL)
        {
            if(strcmp(strTemp,"?") == 0)
            {
                m_pRstEmployees->Fields->GetItem("FaxPhone")->
                    PutValue(vNull);
            }
            else if( (strcmp(strTemp,"X") == 0) | 
                     (strcmp(strTemp,"x") == 0) )
           {
                m_pRstEmployees->Fields->GetItem("FaxPhone")->
                    PutValue("");
            }
            else
            {
                m_pRstEmployees->Fields->GetItem("FaxPhone")->
                    PutValue(strTemp);
            }
            m_pRstEmployees->Update();

            // Open an IADORecordBinding interface pointer which 
            // we will use for binding Recordset to a class
            TESTHR(hr = m_pRstEmployees->QueryInterface(
                __uuidof(IADORecordBinding),(LPVOID*)&picRs));

            // Bind the Recordset to a C++ class here
            TESTHR(hr = picRs->BindToRecordset(&emprs));

            // Print report.
            printf("\nName - Fax number\n");
            printf("%s %s ",emprs.lemp_LastNameStatus == adFldOK ? 
                emprs.m_szemp_LastName : "<NULL>",
                emprs.lemp_FirstNameStatus == adFldOK ? 
                emprs.m_szemp_FirstName : "<NULL>");

            if (emprs.lemp_FaxphoneStatus == adFldNull)
                printf("- [Unknown]");
            else if (strcmp((LPSTR)emprs.m_szemp_Faxphone,"") == 0)
                printf("- [Has no fax]");
            else
                printf("- %s",emprs.m_szemp_Faxphone);
        }
        m_pRstEmployees->Close();

        // Delete new field because this is a demonstration.
        m_pTable->Columns->Delete(m_pColumn->Name);

        // Release the IADORecordset Interface here
        if(picRs)
            picRs->Release();
        m_pCnn->Close();
    }

    catch(_com_error &e)
    {
        // Notify the user of errors if any.
        _bstr_t bstrSource(e.Source());
        _bstr_t bstrDescription(e.Description());
          
        printf("\n\tSource :  %s \n\tdescription : %s \n ",
            (LPCSTR)bstrSource,(LPCSTR)bstrDescription);
    }

    catch(...)
    {
        cout << "Error occured in include files...."<< endl;
    }
}

AttributesX.h

#include "icrsint.h"

//This class extracts LastName, FirstName, FaxPhone from Employees table

class CEmployeeRs : public CADORecordBinding
{
BEGIN_ADO_BINDING(CEmployeeRs)
        // Column LastName is the 2nd field in the table
        ADO_VARIABLE_LENGTH_ENTRY2(2,adVarChar,m_szemp_LastName,
            sizeof(m_szemp_LastName),lemp_LastNameStatus,TRUE)
        // Column FirstName is the 17th field in the table
        ADO_VARIABLE_LENGTH_ENTRY2(17,adVarChar,m_szemp_FirstName,
            sizeof(m_szemp_FirstName),lemp_FirstNameStatus,TRUE)
        // Column FaxPhone is the 18th field in the table
        ADO_VARIABLE_LENGTH_ENTRY2(18,adVarChar,m_szemp_Faxphone,
            sizeof(m_szemp_Faxphone),lemp_FaxphoneStatus,TRUE)
END_ADO_BINDING()

public:
    CHAR m_szemp_LastName[21];
    ULONG lemp_LastNameStatus;
    CHAR m_szemp_FirstName[11];
    ULONG lemp_FirstNameStatus;
    CHAR m_szemp_Faxphone[25];
    ULONG lemp_FaxphoneStatus;
};