Esempio dei metodi Execute, Requery e Clear (VC++)

In questo esempio viene illustrato il metodo Execute quando viene eseguito da un oggetto Command e da un oggetto Connection. Usa inoltre il metodo Requery per recuperare i dati correnti in un Recordset e il metodo Clear per cancellare il contenuto della raccolta Errors. Per l'esecuzione di questo esempio sono necessarie le funzioni ExecuteCommand e PrintOutput.

// Execute_Requery_Clear_Method_Sample.cpp  
// compile with: /EHsc  
#include <ole2.h>  
#include <stdio.h>  
  
#import "msado15.dll" no_namespace rename("EOF", "EndOfFile")  
  
// Function declarations  
inline void TESTHR(HRESULT x) { if FAILED(x) _com_issue_error(x); };  
void ExecuteX();  
void ExecuteCommand(_CommandPtr pCmdTemp, _RecordsetPtr pRstTemp);  
void PrintOutput(_RecordsetPtr pRstTemp);  
void PrintProviderError(_ConnectionPtr pConnection);  
void PrintComError(_com_error &e);  
  
int main() {  
   if ( FAILED(::CoInitialize(NULL)) )  
      return -1;  
  
   ExecuteX();  
   ::CoUninitialize();  
}  
  
void ExecuteX() {  
   // Define string variables.  
   _bstr_t strSQLChange("UPDATE Titles SET Type = 'self_help' WHERE Type = 'psychology'");  
   _bstr_t strSQLRestore("UPDATE Titles SET Type = 'psychology' WHERE Type = 'self_help'");  
   _bstr_t strCnn("Provider='sqloledb'; Data Source='My_Data_Source'; Initial Catalog='pubs'; Integrated Security='SSPI';");  
  
   // Define ADO object pointers.  Initialize pointers on define.  
   // These are in the ADODB::  namespace.  
   _ConnectionPtr pConnection = NULL;  
   _CommandPtr pCmdChange = NULL;  
   _RecordsetPtr pRstTitles = NULL;  
  
   try {  
      // Open connection.  
      TESTHR(pConnection.CreateInstance(__uuidof(Connection)));  
      pConnection->Open (strCnn, "", "", adConnectUnspecified);  
  
      // Create command object.  
      TESTHR(pCmdChange.CreateInstance(__uuidof(Command)));  
      pCmdChange->ActiveConnection = pConnection;  
      pCmdChange->CommandText = strSQLChange;  
  
      // Open titles table, casting Connection pointer to an   
      // IDispatch type so converted to correct type of variant.  
      TESTHR(pRstTitles.CreateInstance(__uuidof(Recordset)));  
      pRstTitles->Open ("Titles", _variant_t((IDispatch *) pConnection, true),   
         adOpenStatic, adLockOptimistic, adCmdTable);  
  
      // Print report of original data.  
      printf("\n\nData in Titles table before executing the query: \n");  
  
      // Call function to print loop recordset contents.  
      PrintOutput(pRstTitles);  
  
      // Clear extraneous errors from the Errors collection.  
      pConnection->Errors->Clear();  
  
      // Call ExecuteCommand subroutine to execute pCmdChange command.  
      ExecuteCommand(pCmdChange, pRstTitles);  
  
      // Print report of new data.  
      printf("\n\n\tData in Titles table after executing the query: \n");  
      PrintOutput(pRstTitles);  
  
      // Use Connection object's Execute method to execute SQL statement to restore data.  
      pConnection->Execute(strSQLRestore, NULL, adExecuteNoRecords);  
  
      // Retrieve the current data by requerying the recordset.  
      pRstTitles->Requery(adCmdUnknown);  
  
      // Print report of restored data.  
      printf("\n\n\tData after exec. query to restore original info: \n");  
      PrintOutput(pRstTitles);  
   }  
   catch (_com_error &e) {  
      PrintProviderError(pConnection);  
      PrintComError(e);  
   }  
  
   // Clean up objects before exit.  
   if (pRstTitles)  
      if (pRstTitles->State == adStateOpen)  
         pRstTitles->Close();  
   if (pConnection)  
      if (pConnection->State == adStateOpen)  
         pConnection->Close();  
}  
  
void ExecuteCommand(_CommandPtr pCmdTemp, _RecordsetPtr pRstTemp) {  
   try {  
      // CommandText property already set before function was called.  
      pCmdTemp->Execute(NULL, NULL, adCmdText);  
  
      // Retrieve the current data by requerying the recordset.  
      pRstTemp->Requery(adCmdUnknown);  
   }  
  
   catch(_com_error &e) {  
      // Notify user of any errors that result from executing the query.  
      // Pass a connection pointer accessed from the Recordset.  
      PrintProviderError(pRstTemp->GetActiveConnection());  
      PrintComError(e);  
   }  
}  
  
void PrintOutput(_RecordsetPtr pRstTemp) {  
   // Ensure at top of recordset.  
   pRstTemp->MoveFirst();  
  
   // If EOF is true, then no data and skip print loop.  
   if ( pRstTemp->EndOfFile )  
      printf("\tRecordset empty\n");  
   else {  
      // Define strings for output conversions.  Initialize to first record's values.  
      _bstr_t bstrTitle;  
      _bstr_t bstrType;  
  
      // Enumerate Recordset and print from each.  
      while ( !(pRstTemp->EndOfFile) ) {  
         // Convert variant string to convertable string type.  
         bstrTitle = pRstTemp->Fields->GetItem("Title")->Value;  
         bstrType  = pRstTemp->Fields->GetItem("Type")->Value;  
         printf("\t%s, %s \n", (LPCSTR) bstrTitle, (LPCSTR) bstrType);  
  
         pRstTemp->MoveNext();  
      }  
   }  
}  
  
void PrintProviderError(_ConnectionPtr pConnection) {  
   // Print Provider Errors from Connection object.  
   // pErr is a record object in the Connection's Error collection.  
   ErrorPtr pErr = NULL;  
  
   if ( (pConnection->Errors->Count) > 0 ) {  
      long nCount = pConnection->Errors->Count;  
      // Collection ranges from 0 to nCount -1.  
      for ( long i = 0 ; i < nCount ; i++ ) {  
         pErr = pConnection->Errors->GetItem(i);  
         printf("\t Error number: %x\t%s", pErr->Number, pErr->Description);  
      }  
   }  
}  
  
void PrintComError(_com_error &e) {  
   _bstr_t bstrSource(e.Source());  
   _bstr_t bstrDescription(e.Description());  
  
   // Print Com errors.  
   printf("Error\n");  
   printf("\tCode = %08lx\n", e.Error());  
   printf("\tCode meaning = %s\n", e.ErrorMessage());  
   printf("\tSource = %s\n", (LPCSTR) bstrSource);  
   printf("\tDescription = %s\n", (LPCSTR) bstrDescription);  
}  

Vedere anche

Metodo Clear (ADO)
Oggetto Command (ADO)
Oggetto Connection (ADO)
Raccolta Errors (ADO)
Metodo Execute (Command - ADO)
Metodo Execute (Connection - ADO)
Metodo Requery