1. 新增WS_HSCROLL樣式

使用記事本開啟.rc資原始檔增加WS_HSCROLL樣式(VS的資源編輯視窗中無法新增)，如下圖所示。

2.新增如下程式碼

void CXXXDlg::AddHScroll(UINT idComboBox)
{
    CComboBox *pCB = (CComboBox*)GetDlgItem(idComboBox);
    if (nullptr == pCB) return;

    CComboBox &Cmb = *pCB;
    // Find the longest string in the combo box.
 

    CString str;
    CSize sz;
    int dx = 0;

    TEXTMETRIC tm = { 0 };
    CDC* pDC = Cmb.GetDC();
    CFont* pFont = Cmb.GetFont();

    // Select the listbox font, save the old font
    CFont* pOldFont = pDC->SelectObject(pFont);

    // Get the text metrics for avg char width
    pDC->GetTextMetrics
 
(&tm);

    for (int i = 0; i < Cmb.GetCount(); i++)
    {
        Cmb.GetLBText(i, str);
        sz = pDC->GetTextExtent(str);

        // Add the avg width to prevent clipping
        sz.cx += tm.tmAveCharWidth;

        if (sz.cx > dx)
            dx = sz.cx;
    }

    // Select the old font back into the DC
 

    pDC->SelectObject(pOldFont);
    Cmb.ReleaseDC(pDC);

    // Set the horizontal extent so every character of all strings can
    // be scrolled to.
    Cmb.SetHorizontalExtent(dx);
}

3.在OnInitDialog中呼叫函式設定水平滾動資訊

BOOL CXXXDlg::OnInitDialog()
{
	//---
	AddHScroll(IDC_COMBO_L_LL_CMD);
	//---
}

參考文章:http://ntcoder.com/bab/2009/05/28/add-horizontal-scrollbar-to-a-combo-box/