Cancel 方法范例 (VC++/ADO)

 

本范例使用 Cancel 方法，在连接繁忙时取消在 Connection 对象上执行的命令。

#import "C:/Program Files/Common Files/System/ADO/msado15.dll" /    no_namespace rename("EOF", "EndOfFile")#include <ole2.h>#include <stdio.h>#include<conio.h>// Function declarationsinline void TESTHR(HRESULT x) {if FAILED(x) com_issue_error(x);};void CancelX();void PrintProviderError(_ConnectionPtr pConnection);void PrintComError(_com_error &e);/////////////////////////////////////////////////////////////                                                       ////      Main Function                                    ////                                                       /////////////////////////////////////////////////////////////void main(){    if(FAILED(::CoInitialize(NULL)))        return;    CancelX();    ::CoUninitialize();    }}/////////////////////////////////////////////////////////////                                                       ////      CancelX Function                                 ////                                                       /////////////////////////////////////////////////////////////void CancelX(){    // Define ADO object pointers.    // Initialize pointers on define.    // These are in the ADODB::  namespace     _ConnectionPtr pConnection = NULL;     //Define Other Variables    HRESULT  hr = S_OK;    _bstr_t strCmdChange;    _bstr_t strCmdRestore;    BOOL booChanged = FALSE;    _bstr_t strCnn("Provider=sqloledb;Data Source=srv;"    "Initial Catalog=Pubs;User Id=sa;Password=;");    try    {        // open a connection.        TESTHR(pConnection.CreateInstance(__uuidof(Connection)));        pConnection->Open(strCnn,"","",NULL);        // 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<=10 ;i++)         {            i = i + 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()) && (adStateExecuting))        {            pConnection->Cancel();            pConnection->RollbackTrans();            booChanged = FALSE;            printf("Update canceled./n");        }        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");        }        // Cleanup object before exit            pConnection->Close ();    }    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);    }}/////////////////////////////////////////////////////////////                                                       ////      PrintProviderError Function                      ////                                                       /////////////////////////////////////////////////////////////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/n", pErr->Number,                pErr->Description);        }    }}/////////////////////////////////////////////////////////////                                                       ////      PrintComError Function                           ////                                                       /////////////////////////////////////////////////////////////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);}