Cancel-Methode – Beispiel (VC++)
In diesem Beispiel wird die Cancel-Methode verwendet, um einen Befehl abzubrechen, der auf einem Connection-Objekt ausgeführt wird, wenn die Verbindung stark ausgelastet ist.
// CancelMethodExample.cpp
// compile with: /EHsc
#import "msado15.dll" no_namespace rename("EOF", "EndOfFile")
#include <ole2.h>
#include <stdio.h>
#include<conio.h>
// Function declarations
inline void TESTHR(HRESULT x) { if FAILED(x) _com_issue_error(x); };
void CancelX();
void PrintProviderError(_ConnectionPtr pConnection);
void PrintComError(_com_error &e);
int main() {
if ( FAILED(::CoInitialize(NULL)) )
return -1;
CancelX();
::CoUninitialize();
}
void CancelX() {
// Define ADO object pointers. Initialize pointers on define.
// These are in the ADODB:: namespace
_ConnectionPtr pConnection = NULL;
// Define Other Variables
_bstr_t strCmdChange;
_bstr_t strCmdRestore;
BOOL booChanged = FALSE;
_bstr_t strCnn("Provider='sqloledb'; Data Source='My_Data_Source'; Initial Catalog='pubs'; Integrated Security='SSPI';");
try {
// open a connection.
TESTHR(pConnection.CreateInstance(__uuidof(Connection)));
pConnection->Open(strCnn, "", "", adConnectUnspecified);
// Define command strings.
strCmdChange = "UPDATE titles SET type = 'self_help' WHERE type = 'psychology'";
strCmdRestore = "UPDATE titles SET type = 'psychology' WHERE type = 'self_help'";
// Begin a transaction, then execute a command asynchronously.
pConnection->BeginTrans();
pConnection->Execute(strCmdChange, NULL, adAsyncExecute);
// do something else for a little while - this could be changed
for ( int i = 1 ; i <= 100000 ; i++ )
printf("%d\n", i);
// If the command has NOT completed, cancel the execute and roll back the
// transaction. Otherwise, commit the transaction.
if ( (pConnection->GetState()) ) {
pConnection->Cancel();
pConnection->RollbackTrans();
booChanged = FALSE;
printf("Update canceled.\n");
printf("GetState = %d\n", pConnection->GetState());
}
else {
pConnection->CommitTrans();
booChanged = TRUE;
printf("Update complete.\n");
}
// If the change was made, restore the data because this is a demonstration.
if (booChanged) {
pConnection->Execute(strCmdRestore, NULL, 0);
printf("Data restored.\n");
}
}
catch(_com_error &e) {
// Notify user of any errors that result from executing the query.
// Pass a connection pointer accessed from the Connection.
PrintProviderError(pConnection);
PrintComError(e);
}
// Cleanup object before exit
if (pConnection)
if (pConnection->State == adStateOpen)
pConnection->Close();
}
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("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);
}