Attributes-Eigenschaft (VC++-Beispiel)

Gilt für: Access 2013, Office 2013

This example demonstrates the Attributes property of a Column. Setting it to adColNullable allows the user to set the value of a RecordsetField 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.

 
// BeginAttributesCpp 
#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 "ADOXAttributesX.h" 
 
//Function declarations 
inline void TESTHR(HRESULT x) {if FAILED(x) _com_issue_error(x);}; 
void AttributesX(void); 
inline char* mygets(char* strDest, int n) 
{ 
 char strExBuff[10]; 
 char* pstrRet = fgets(strDest, n, stdin); 
 
 if (pstrRet == NULL) 
 return NULL; 
 
 if (!strrchr(strDest, '\n')) 
 // Exhaust the input buffer. 
 do 
 { 
 fgets(strExBuff, sizeof(strExBuff), stdin); 
 }while (!strrchr(strExBuff, '\n')); 
 else 
 // Replace '\n' with '\0' 
 strDest[strrchr(strDest, '\n') - strDest] = '\0'; 
 
 return pstrRet; 
} 
 
////////////////////////////////////////////////////////// 
// // 
// 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(m_pColumn->Name, adVarWChar, 24); 
 //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]; 
 mygets(strInput, 10); 
 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]\n"); 
 else if (strcmp((LPSTR)emprs.m_szemp_Faxphone,"") == 0) 
 printf("- [Has no fax]\n"); 
 else 
 printf("- %s\n",emprs.m_szemp_Faxphone); 
 
 } 
 
 // Delete new field because this is a demonstration. 
 //m_pTable->Columns->Delete(m_pColumn->Name); 
 } 
 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 AttributesX...."<< endl; 
 } 
 
 
 if (m_pRstEmployees) 
 if (m_pRstEmployees->State == 1) 
 m_pRstEmployees->Close(); 
 
 // Delete new field because this is a demonstration. 
 if (m_pTable != NULL) 
 m_pTable->Columns->Delete(m_pColumn->Name); 
 
 if (m_pCnn) 
 if (m_pCnn->State == 1) 
 m_pCnn->Close(); 
 
 // Release the IADORecordset Interface here 
 if(picRs) 
 picRs->Release(); 
 
} 
// EndAttributesCpp 

ADOXAttributesX.h

    // BeginAttributesH 
    #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; 
    }; 
    // EndAttributesH