多執行緒 DAO 程式設計：GUI 執行緒封送與 CMultiDaoRecordSet 實作模式

編輯：關於C語言

DAO（Data Access Objects）在多執行緒環境下有嚴格限制：DAO 物件必須在建立它的執行緒中使用，跨執行緒直接呼叫會導致未定義行為或崩潰。本文提出一種以 GUI 執行緒封送 為核心的解決方案，封裝 CMultiDaoRecordSet 基類，透過視窗訊息將背景執行緒的 DAO 操作請求轉發至主執行緒執行，確保執行緒親和性並簡化上層邏輯。

DAO 執行緒親和性限制

DAO 內部使用單執行緒 COM 單元（STA），物件綁定建立時的執行緒。
背景執行緒直接操作 CDaoRecordset 會觸發 DAO 3609 錯誤或存取違規。
MFC CDaoDatabase/CDaoRecordset 無內建跨執行緒封送機制。

核心架構：訊息驅動封送模式

sequenceDiagram
    participant Worker as 背景執行緒
    participant Main as GUI 主執行緒
    participant DAO as CDaoRecordset

    Worker->>Main: PostMessage(WM_MULTIDAOMESSAGE, cmd, pRecordset)
    Main->>Main: OnMultiDaoMessage() 接收
    Main->>DAO: 執行實際 DAO 操作 (Open/Edit/Update/Delete)
    DAO-->>Main: 回傳結果/錯誤碼
    Main->>Worker: PostThreadMessage / 事件通知完成

關鍵類別實作

1. 自訂訊息定義

// MultiDaoRecordset.h
#define WM_MULTIDAOMESSAGE RegisterWindowMessage(_T("WM_MULTIDAOMESSAGE"))

enum MultiDaoCmd {
    DAO_CMD_OPEN,
    DAO_CMD_CLOSE,
    DAO_CMD_MOVE_NEXT,
    DAO_CMD_MOVE_PREV,
    DAO_CMD_EDIT,
    DAO_CMD_UPDATE,
    DAO_CMD_DELETE,
    DAO_CMD_ADDNEW,
    DAO_CMD_FIND,
    DAO_CMD_REQUERY
};

struct MultiDaoParam {
    MultiDaoCmd cmd;
    CMultiDaoRecordSet* pSet;
    COleVariant var1, var2; // 通用參數
    LONG lResult;           // 執行結果
    CString strError;       // 錯誤訊息
    HANDLE hEvent;          // 同步事件（可選）
};

2. 基類 CMultiDaoRecordSet

class CMultiDaoRecordSet : public CDaoRecordset
{
    DECLARE_DYNAMIC(CMultiDaoRecordSet)

public:
    CMultiDaoRecordSet(CDaoDatabase* pdb = NULL) : CDaoRecordset(pdb) {}

    // 覆寫關鍵虛擬函式，攔截呼叫並轉發至 GUI 執行緒
    virtual BOOL Open(int nOpenType = dbOpenDynaset, LPCTSTR lpszSQL = NULL, int nOptions = 0) {
        return MarshalCall(DAO_CMD_OPEN, (LPARAM)lpszSQL, nOptions);
    }
    virtual void Close() { MarshalCall(DAO_CMD_CLOSE); }
    virtual BOOL MoveNext() { return (BOOL)MarshalCall(DAO_CMD_MOVE_NEXT); }
    virtual BOOL MovePrev() { return (BOOL)MarshalCall(DAO_CMD_MOVE_PREV); }
    virtual void Edit() { MarshalCall(DAO_CMD_EDIT); }
    virtual void Update() { MarshalCall(DAO_CMD_UPDATE); }
    virtual void Delete() { MarshalCall(DAO_CMD_DELETE); }
    virtual void AddNew() { MarshalCall(DAO_CMD_ADDNEW); }

protected:
    LONG MarshalCall(MultiDaoCmd cmd, LPARAM param1 = 0, LPARAM param2 = 0) {
        // 檢查是否已在 GUI 執行緒
        if (AfxGetThread() == AfxGetApp()->m_pMainWnd->GetWindowThreadProcessId()) {
            // 直接執行基類實作
            return DirectCall(cmd, param1, param2);
        }

        // 封裝參數
        MultiDaoParam params;
        params.cmd = cmd;
        params.pSet = this;
        params.var1 = param1; params.var2 = param2;
        params.hEvent = CreateEvent(NULL, TRUE, FALSE, NULL);

        // 發送至主視窗
        ::PostMessage(AfxGetMainWnd()->GetSafeHwnd(), WM_MULTIDAOMESSAGE, 0, (LPARAM)¶ms);

        // 等待完成（可選：非同步模式則不等待，改用回調）
        WaitForSingleObject(params.hEvent, INFINITE);
        CloseHandle(params.hEvent);

        if (!params.strError.IsEmpty())
            AfxThrowDaoException(params.lResult, params.strError);
        return params.lResult;
    }

    LONG DirectCall(MultiDaoCmd cmd, LPARAM p1, LPARAM p2) {
        switch (cmd) {
            case DAO_CMD_OPEN:  return CDaoRecordset::Open((int)p1, (LPCTSTR)p2);
            case DAO_CMD_CLOSE: CDaoRecordset::Close(); return 0;
            case DAO_CMD_MOVE_NEXT: return CDaoRecordset::MoveNext();
            // ... 其他命令
        }
        return 0;
    }
};
IMPLEMENT_DYNAMIC(CMultiDaoRecordSet, CDaoRecordset)

3. 主視窗訊息處理

// MainFrm.h
#define MAINFRAME_CPP
#include "MultiDaoRecordset.h"
UINT WM_MULTIDAOMESSAGE = RegisterWindowMessage(_T("WM_MULTIDAOMESSAGE"));

class CMainFrame : public CMDIFrameWnd {
    // ...
    afx_msg LONG OnMultiDaoMessage(UINT uParam, LONG lParam);
};

// MainFrm.cpp
BEGIN_MESSAGE_MAP(CMainFrame, CMDIFrameWnd)
    ON_REGISTERED_MESSAGE(WM_MULTIDAOMESSAGE, OnMultiDaoMessage)
END_MESSAGE_MAP()

LONG CMainFrame::OnMultiDaoMessage(UINT uParam, LONG lParam)
{
    MultiDaoParam* pParams = (MultiDaoParam*)lParam;
    CMultiDaoRecordSet* pSet = pParams->pSet;
    LONG lRet = 0;
    CString strErr;

    try {
        switch (pParams->cmd) {
            case DAO_CMD_OPEN:
                lRet = pSet->CDaoRecordset::Open((int)pParams->var1.lVal,
                                                 (LPCTSTR)pParams->var1.pbstrVal,
                                                 (int)pParams->var2.lVal);
                break;
            case DAO_CMD_CLOSE:
                pSet->CDaoRecordset::Close(); break;
            case DAO_CMD_MOVE_NEXT:
                lRet = pSet->CDaoRecordset::MoveNext(); break;
            case DAO_CMD_MOVE_PREV:
                lRet = pSet->CDaoRecordset::MovePrev(); break;
            case DAO_CMD_EDIT:
                pSet->CDaoRecordset::Edit(); break;
            case DAO_CMD_UPDATE:
                pSet->CDaoRecordset::Update(); break;
            case DAO_CMD_DELETE:
                pSet->CDaoRecordset::Delete(); break;
            case DAO_CMD_ADDNEW:
                pSet->CDaoRecordset::AddNew(); break;
            // ... 其他命令
        }
    } catch (CDaoException* e) {
        lRet = e->m_nAfxDaoError;
        strErr = e->m_strError;
        e->Delete();
    } catch (CException* e) {
        TCHAR buf[256]; e->GetErrorMessage(buf, 256);
        strErr = buf; lRet = -1;
        e->Delete();
    }

    pParams->lResult = lRet;
    pParams->strError = strErr;
    SetEvent(pParams->hEvent); // 通知呼叫執行緒完成
    return 0;
}

4. 衍生類別使用範例

// MySet.h
class CMySet : public CMultiDaoRecordSet {
    DECLARE_DYNAMIC(CMySet)
public:
    CMySet(CDaoDatabase* pdb = NULL) : CMultiDaoRecordSet(pdb) {}
    // 宣告欄位...
};

// MySet.cpp
IMPLEMENT_DYNAMIC(CMySet, CMultiDaoRecordSet)
CMySet::CMySet(CDaoDatabase* pdb) : CMultiDaoRecordSet(pdb) {}

// 背景執行緒中安全使用
UINT WorkerThread(LPVOID pParam) {
    CMySet set(&db);
    set.Open();           // 自動封送至 GUI 執行緒
    while (!set.IsEOF()) {
        // 讀取欄位...
        set.MoveNext();   // 自動封送
    }
    set.Close();
    return 0;
}