OriginalValue 和 UnderlyingValue 属性示例 (VC++)
此示例演示 OriginalValue 和 UnderlyingValue 属性，方法是在记录的基础数据于 Recordset 批量更新期间更改时显示消息。
示例
// BeginOriginalValueCpp.cpp
// compile with: /EHsc
#import "msado15.dll" no_namespace rename("EOF", "EndOfFile")
#include <ole2.h>
#include <stdio.h>
#include <conio.h>
#include "icrsint.h"
// class extracts title_id and type from titles table
class CTitleRs : public CADORecordBinding {
BEGIN_ADO_BINDING(CTitleRs)
// Column title_id is the 1st field in the Recordset
ADO_VARIABLE_LENGTH_ENTRY2(1, adVarChar, m_szau_Title_id, sizeof(m_szau_Title_id), lau_Title_idStatus, FALSE)
// Column type is the 3rd field in the Recordset
ADO_VARIABLE_LENGTH_ENTRY2(3, adVarChar, m_szau_Type, sizeof(m_szau_Type), lau_TypeStatus, TRUE)
END_ADO_BINDING()
public:
CHAR m_szau_Title_id[7];
ULONG lau_Title_idStatus;
CHAR m_szau_Type[13];
ULONG lau_TypeStatus;
};
// Function declarations
inline void TESTHR(HRESULT x) { if FAILED(x) _com_issue_error(x); };
void OriginalValueX();
void PrintProviderError(_ConnectionPtr pConnection);
void PrintComError(_com_error &e);
int main() {
if (FAILED(::CoInitialize(NULL)))
return -1;
OriginalValueX();
::CoUninitialize();
}
void OriginalValueX() {
// Define ADO object pointers, initialize pointers on define. These are in the ADODB:: namespace.
_ConnectionPtr pConnection = NULL;
FieldPtr pFldType = NULL;
_RecordsetPtr pRstTitles = NULL;
// Define string variables.
_bstr_t strSQLChange("UPDATE Titles SET Type = 'sociology' WHERE Type = 'psychology'");
_bstr_t strSQLRestore("UPDATE Titles SET Type = 'psychology' WHERE Type = 'sociology'");
// Define Other Variables
HRESULT hr = S_OK;
IADORecordBinding *picRs = NULL; // Interface Pointer declared
CTitleRs titlers; // C++ Class object
char * token1;
try {
_bstr_t strCnn("Provider='sqloledb'; Data Source='My_Data_Source'; Initial Catalog='pubs'; Integrated Security='SSPI';");
// Open connection.
TESTHR(pConnection.CreateInstance(__uuidof(Connection)));
pConnection->Open (strCnn, "", "", adConnectUnspecified);
// Open Recordset for batch update.
TESTHR(pRstTitles.CreateInstance(__uuidof(Recordset)));
pRstTitles->PutActiveConnection(_variant_t((IDispatch *)pConnection, true));
pRstTitles->CursorType = adOpenKeyset;
pRstTitles->LockType = adLockBatchOptimistic;
// Cast connection pointer to IDispatch type; convert to correct variant type.
pRstTitles->Open("Titles", _variant_t((IDispatch *)pConnection,true),
adOpenKeyset, adLockBatchOptimistic, adCmdTable);
// Open IADORecordBinding interface pointer for Binding Recordset to a class.
TESTHR(pRstTitles->QueryInterface(__uuidof(IADORecordBinding), (LPVOID*)&picRs));
// Bind the Recordset to a C++ Class here
TESTHR(picRs->BindToRecordset(&titlers));
// Set field object variable for Type field.
pFldType = pRstTitles->Fields->GetItem("type");
// Change the type of psychology titles.
while ( !(pRstTitles->EndOfFile) ) {
if (!strcmp(strtok_s((char *)titlers.m_szau_Type, " ", &token1), "psychology"))
pFldType->Value = "self_help";
pRstTitles->MoveNext();
}
// Simulate a change by another user by updating data using a command string.
pConnection->Execute(strSQLChange, NULL, 0);
// Check for changes.
pRstTitles->MoveFirst();
while (!(pRstTitles->EndOfFile)) {
if (strcmp(pFldType->OriginalValue.pcVal, pFldType->UnderlyingValue.pcVal)) {
printf("Data has changed!");
printf("\nTitle ID: %s", titlers.lau_Title_idStatus ==
adFldOK ? titlers.m_szau_Title_id : "<NULL>");
printf("\nCurrent Value: %s", (LPCSTR) (_bstr_t) pFldType->Value);
printf("\nOriginal Value: %s", (LPCSTR) (_bstr_t) pFldType->OriginalValue);
printf("\nUnderlying Value: %s\n\n", (LPCSTR) (_bstr_t) pFldType->UnderlyingValue);
}
pRstTitles->MoveNext();
}
}
catch (_com_error &e) {
// Notify the user of errors if any.
// Pass a connection pointer accessed from the Connection.
PrintProviderError(pConnection);
PrintComError(e);
}
// Clean up objects before exit. Release the IADORecordset Interface here
if (picRs)
picRs->Release();
if (pRstTitles)
if (pRstTitles->State == adStateOpen) {
// Cancel the update because this is a demonstration.
pRstTitles->CancelBatch(adAffectAll);
pRstTitles->Close();
}
if (pConnection)
if (pConnection->State == adStateOpen) {
// Restore Original Values.
pConnection->Execute(strSQLRestore, NULL, 0);
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("\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);
}
数据已更改！
标题 ID：PS1372
当前值：self_help
原始值：psychology
基础值：sociology
数据已更改！
标题 ID：PS2091
当前值：self_help
原始值：psychology
基础值：sociology
数据已更改！
标题 ID：PS2106
当前值：self_help
原始值：psychology
基础值：sociology
数据已更改！
标题 ID：PS3333
当前值：self_help
原始值：psychology
基础值：sociology
数据已更改！
标题 ID：PS7777
当前值：self_help
原始值：psychology
基础值：sociology