https://bbs.csdn.net/topics/391853844
出现的问题和这个网址中的问题一样。
但是在查询学生信息的类中（Find.cpp），对Buttom1定义如下

会出现m_List未定义的情况。
就算把列表所在的头文件加进去也不起作用。
如何解决？
希望各位能详细解答

给你完整写了一个
方法是用构造函数把主窗体传进去
子窗口用 GetParent 获取

效果

采纳本回答，留下email，给你

其他人如果也需要 https://download.csdn.net/download/caozhy/11242831

// Q765982Dlg.cpp : implementation file
//

#include "stdafx.h"
#include "Q765982.h"
#include "Q765982Dlg.h"
#include "FindDialog.h"

#ifdef _DEBUG
#define new DEBUG_NEW
#undef THIS_FILE
static char THIS_FILE[] = __FILE__;
#endif

/////////////////////////////////////////////////////////////////////////////
// CAboutDlg dialog used for App About

class CAboutDlg : public CDialog
{
public:
    CAboutDlg();

// Dialog Data
    //{{AFX_DATA(CAboutDlg)
    enum { IDD = IDD_ABOUTBOX };
    //}}AFX_DATA

    // ClassWizard generated virtual function overrides
    //{{AFX_VIRTUAL(CAboutDlg)
    protected:
    virtual void DoDataExchange(CDataExchange* pDX);    // DDX/DDV support
    //}}AFX_VIRTUAL

// Implementation
protected:
    //{{AFX_MSG(CAboutDlg)
    //}}AFX_MSG
    DECLARE_MESSAGE_MAP()
};

CAboutDlg::CAboutDlg() : CDialog(CAboutDlg::IDD)
{
    //{{AFX_DATA_INIT(CAboutDlg)
    //}}AFX_DATA_INIT
}

void CAboutDlg::DoDataExchange(CDataExchange* pDX)
{
    CDialog::DoDataExchange(pDX);
    //{{AFX_DATA_MAP(CAboutDlg)
    //}}AFX_DATA_MAP
}

BEGIN_MESSAGE_MAP(CAboutDlg, CDialog)
    //{{AFX_MSG_MAP(CAboutDlg)
        // No message handlers
    //}}AFX_MSG_MAP
END_MESSAGE_MAP()

/////////////////////////////////////////////////////////////////////////////
// CQ765982Dlg dialog

CQ765982Dlg::CQ765982Dlg(CWnd* pParent /*=NULL*/)
    : CDialog(CQ765982Dlg::IDD, pParent)
{
    //{{AFX_DATA_INIT(CQ765982Dlg)
        // NOTE: the ClassWizard will add member initialization here
    //}}AFX_DATA_INIT
    // Note that LoadIcon does not require a subsequent DestroyIcon in Win32
    m_hIcon = AfxGetApp()->LoadIcon(IDR_MAINFRAME);
}

void CQ765982Dlg::DoDataExchange(CDataExchange* pDX)
{
    CDialog::DoDataExchange(pDX);
    //{{AFX_DATA_MAP(CQ765982Dlg)
    DDX_Control(pDX, IDC_LIST1, m_list);
    //}}AFX_DATA_MAP
}

BEGIN_MESSAGE_MAP(CQ765982Dlg, CDialog)
    //{{AFX_MSG_MAP(CQ765982Dlg)
    ON_WM_SYSCOMMAND()
    ON_WM_PAINT()
    ON_WM_QUERYDRAGICON()
    ON_BN_CLICKED(IDC_BUTTON1, OnButton1)
    //}}AFX_MSG_MAP
END_MESSAGE_MAP()

/////////////////////////////////////////////////////////////////////////////
// CQ765982Dlg message handlers

BOOL CQ765982Dlg::OnInitDialog()
{
    CDialog::OnInitDialog();

    // Add "About..." menu item to system menu.

    // IDM_ABOUTBOX must be in the system command range.
    ASSERT((IDM_ABOUTBOX & 0xFFF0) == IDM_ABOUTBOX);
    ASSERT(IDM_ABOUTBOX < 0xF000);

    CMenu* pSysMenu = GetSystemMenu(FALSE);
    if (pSysMenu != NULL)
    {
        CString strAboutMenu;
        strAboutMenu.LoadString(IDS_ABOUTBOX);
        if (!strAboutMenu.IsEmpty())
        {
            pSysMenu->AppendMenu(MF_SEPARATOR);
            pSysMenu->AppendMenu(MF_STRING, IDM_ABOUTBOX, strAboutMenu);
        }
    }

    // Set the icon for this dialog.  The framework does this automatically
    //  when the application's main window is not a dialog
    SetIcon(m_hIcon, TRUE);         // Set big icon
    SetIcon(m_hIcon, FALSE);        // Set small icon

    // TODO: Add extra initialization here
    m_list.InsertColumn(0, "ID", LVCFMT_LEFT, 100);
    CString str;
    for (int i = 1; i <= 123; i++)
    {
        str.Format("%d", i);
        m_list.InsertItem(i, (LPCTSTR)str);
    }
    return TRUE;  // return TRUE  unless you set the focus to a control
}

void CQ765982Dlg::OnSysCommand(UINT nID, LPARAM lParam)
{
    if ((nID & 0xFFF0) == IDM_ABOUTBOX)
    {
        CAboutDlg dlgAbout;
        dlgAbout.DoModal();
    }
    else
    {
        CDialog::OnSysCommand(nID, lParam);
    }
}

// If you add a minimize button to your dialog, you will need the code below
//  to draw the icon.  For MFC applications using the document/view model,
//  this is automatically done for you by the framework.

void CQ765982Dlg::OnPaint() 
{
    if (IsIconic())
    {
        CPaintDC dc(this); // device context for painting

        SendMessage(WM_ICONERASEBKGND, (WPARAM) dc.GetSafeHdc(), 0);

        // Center icon in client rectangle
        int cxIcon = GetSystemMetrics(SM_CXICON);
        int cyIcon = GetSystemMetrics(SM_CYICON);
        CRect rect;
        GetClientRect(&rect);
        int x = (rect.Width() - cxIcon + 1) / 2;
        int y = (rect.Height() - cyIcon + 1) / 2;

        // Draw the icon
        dc.DrawIcon(x, y, m_hIcon);
    }
    else
    {
        CDialog::OnPaint();
    }
}

// The system calls this to obtain the cursor to display while the user drags
//  the minimized window.
HCURSOR CQ765982Dlg::OnQueryDragIcon()
{
    return (HCURSOR) m_hIcon;
}

void CQ765982Dlg::OnButton1() 
{
    // TODO: Add your control notification handler code here
    CFindDialog dlg(this); //在这里传给子窗口
    dlg.DoModal();
}

// FindDialog.cpp : implementation file
//

#include "stdafx.h"
#include "Q765982.h"
#include "FindDialog.h"
#include "Q765982Dlg.h"
#ifdef _DEBUG
#define new DEBUG_NEW
#undef THIS_FILE
static char THIS_FILE[] = __FILE__;
#endif

/////////////////////////////////////////////////////////////////////////////
// CFindDialog dialog


CFindDialog::CFindDialog(CWnd* pParent /*=NULL*/)
    : CDialog(CFindDialog::IDD, pParent)
{
    //{{AFX_DATA_INIT(CFindDialog)
        // NOTE: the ClassWizard will add member initialization here
    //}}AFX_DATA_INIT
}


void CFindDialog::DoDataExchange(CDataExchange* pDX)
{
    CDialog::DoDataExchange(pDX);
    //{{AFX_DATA_MAP(CFindDialog)
        // NOTE: the ClassWizard will add DDX and DDV calls here
    //}}AFX_DATA_MAP
}


BEGIN_MESSAGE_MAP(CFindDialog, CDialog)
    //{{AFX_MSG_MAP(CFindDialog)
    ON_BN_CLICKED(IDC_BUTTON1, OnButton1)
    //}}AFX_MSG_MAP
END_MESSAGE_MAP()

/////////////////////////////////////////////////////////////////////////////
// CFindDialog message handlers

void CFindDialog::OnButton1() 
{
    // TODO: Add your control notification handler code here
    CQ765982Dlg * dlg = (CQ765982Dlg *)GetParent(); //得到主窗口
    int i = dlg->m_list.GetItemCount();
    SetDlgItemInt(IDC_EDIT1, i);
}