How to create a resizable CDialog in MFC?

Question

In the RC resource file if the dialog has this style similar to this it will be fixed size:

IDD_DIALOG_DIALOG DIALOGEX 0, 0, 320, 201
STYLE DS_MODALFRAME | WS_POPUP | WS_CAPTION | WS_SYSMENU

If the dialog has this style it will be sizeable:

IDD_DIALOG_DIALOG DIALOGEX 0, 0, 320, 201
STYLE WS_POPUP | WS_VISIBLE | WS_CAPTION | WS_SYSMENU | WS_THICKFRAME

With these sizable frame options the dialog will be re-sizeable but you will still need to do a lot of work handling the WM_SIZE message to manage the sizing an positioning of the controls within the dialog.

In addition to setting the style to WS_THICKFRAME, you'll probably also want to have a system to move and resize the controls in a dialog as the dialog is resized. For my own personal use I've created a base class to replace CDialog that has this capability. Derive from this class and in your InitDialog function call the AutoMove function for each child control to define how much it should move and how much it should resize relative to the parent dialog. The size of the dialog in the resource file is used as a minimum size.

BaseDialog.h:

#if !defined(AFX_BASEDIALOG_H__DF4DE489_4474_4759_A14E_EB3FF0CDFBDA__INCLUDED_)
#define AFX_BASEDIALOG_H__DF4DE489_4474_4759_A14E_EB3FF0CDFBDA__INCLUDED_

#if _MSC_VER > 1000
#pragma once
#endif // _MSC_VER > 1000

#include <vector>

class CBaseDialog : public CDialog
{
// Construction
public:
    CBaseDialog(UINT nIDTemplate, CWnd* pParent = NULL);   // standard constructor

    void AutoMove(int iID, double dXMovePct, double dYMovePct, double dXSizePct, double dYSizePct);

// Overrides
    // ClassWizard generated virtual function overrides
    //{{AFX_VIRTUAL(CBaseDialog)
protected:
    //}}AFX_VIRTUAL

protected:
    //{{AFX_MSG(CBaseDialog)
    virtual BOOL OnInitDialog();
    afx_msg void OnGetMinMaxInfo(MINMAXINFO FAR* lpMMI);
    afx_msg void OnSize(UINT nType, int cx, int cy);
    //}}AFX_MSG
    DECLARE_MESSAGE_MAP()

public:
    bool            m_bShowGripper;         // ignored if not WS_THICKFRAME

private:
    struct SMovingChild
    {
        HWND        m_hWnd;
        double      m_dXMoveFrac;
        double      m_dYMoveFrac;
        double      m_dXSizeFrac;
        double      m_dYSizeFrac;
        CRect       m_rcInitial;
    };
    typedef std::vector<SMovingChild>   MovingChildren;

    MovingChildren  m_MovingChildren;
    CSize           m_szInitial;
    CSize           m_szMinimum;
    HWND            m_hGripper;
};

//{{AFX_INSERT_LOCATION}}
// Microsoft Visual C++ will insert additional declarations immediately before the previous line.

#endif // !defined(AFX_BASEDIALOG_H__DF4DE489_4474_4759_A14E_EB3FF0CDFBDA__INCLUDED_)

BaseDialog.cpp:

#include "stdafx.h"
#include "BaseDialog.h"

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

CBaseDialog::CBaseDialog(UINT nIDTemplate, CWnd* pParent /*=NULL*/)
    : CDialog(nIDTemplate, pParent),
      m_bShowGripper(true),
      m_szMinimum(0, 0),
      m_hGripper(NULL)
{
}


BEGIN_MESSAGE_MAP(CBaseDialog, CDialog)
    //{{AFX_MSG_MAP(CBaseDialog)
    ON_WM_GETMINMAXINFO()
    ON_WM_SIZE()
    //}}AFX_MSG_MAP
END_MESSAGE_MAP()

void CBaseDialog::AutoMove(int iID, double dXMovePct, double dYMovePct, double dXSizePct, double dYSizePct)
{
    ASSERT((dXMovePct + dXSizePct) <= 100.0);   // can't use more than 100% of the resize for the child
    ASSERT((dYMovePct + dYSizePct) <= 100.0);   // can't use more than 100% of the resize for the child
    SMovingChild s;
    GetDlgItem(iID, &s.m_hWnd);
    ASSERT(s.m_hWnd != NULL);
    s.m_dXMoveFrac = dXMovePct / 100.0;
    s.m_dYMoveFrac = dYMovePct / 100.0;
    s.m_dXSizeFrac = dXSizePct / 100.0;
    s.m_dYSizeFrac = dYSizePct / 100.0;
    ::GetWindowRect(s.m_hWnd, &s.m_rcInitial);
    ScreenToClient(s.m_rcInitial);
    m_MovingChildren.push_back(s);
}

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

    // use the initial dialog size as the default minimum
    if ((m_szMinimum.cx == 0) && (m_szMinimum.cy == 0))
    {
        CRect rcWindow;
        GetWindowRect(rcWindow);
        m_szMinimum = rcWindow.Size();
    }

    // keep the initial size of the client area as a baseline for moving/sizing controls
    CRect rcClient;
    GetClientRect(rcClient);
    m_szInitial = rcClient.Size();

    // create a gripper in the bottom-right corner
    if (m_bShowGripper && ((GetStyle() & WS_THICKFRAME) != 0))
    {
        SMovingChild s;
        s.m_rcInitial.SetRect(-GetSystemMetrics(SM_CXVSCROLL), -GetSystemMetrics(SM_CYHSCROLL), 0, 0);
        s.m_rcInitial.OffsetRect(rcClient.BottomRight());
        m_hGripper = CreateWindow(_T("Scrollbar"), _T("size"), WS_CHILD | WS_VISIBLE | SBS_SIZEGRIP,
                                  s.m_rcInitial.left, s.m_rcInitial.top, s.m_rcInitial.Width(), s.m_rcInitial.Height(),
                                  m_hWnd, NULL, AfxGetInstanceHandle(), NULL);
        ASSERT(m_hGripper != NULL);
        if (m_hGripper != NULL)
        {
            s.m_hWnd = m_hGripper;
            s.m_dXMoveFrac = 1.0;
            s.m_dYMoveFrac = 1.0;
            s.m_dXSizeFrac = 0.0;
            s.m_dYSizeFrac = 0.0;
            m_MovingChildren.push_back(s);

            // put the gripper first in the z-order so it paints first and doesn't obscure other controls
            ::SetWindowPos(m_hGripper, HWND_TOP, 0, 0, 0, 0, SWP_NOACTIVATE | SWP_NOMOVE | SWP_NOOWNERZORDER | SWP_NOSIZE | SWP_SHOWWINDOW);
        }
    }

    return TRUE;  // return TRUE  unless you set the focus to a control
}

void CBaseDialog::OnGetMinMaxInfo(MINMAXINFO FAR* lpMMI) 
{
    CDialog::OnGetMinMaxInfo(lpMMI);

    if (lpMMI->ptMinTrackSize.x < m_szMinimum.cx)
        lpMMI->ptMinTrackSize.x = m_szMinimum.cx;
    if (lpMMI->ptMinTrackSize.y < m_szMinimum.cy)
        lpMMI->ptMinTrackSize.y = m_szMinimum.cy;
}

void CBaseDialog::OnSize(UINT nType, int cx, int cy) 
{
    CDialog::OnSize(nType, cx, cy);

    int iXDelta = cx - m_szInitial.cx;
    int iYDelta = cy - m_szInitial.cy;
    HDWP hDefer = NULL;
    for (MovingChildren::iterator p = m_MovingChildren.begin();  p != m_MovingChildren.end();  ++p)
    {
        if (p->m_hWnd != NULL)
        {
            CRect rcNew(p->m_rcInitial);
            rcNew.OffsetRect(int(iXDelta * p->m_dXMoveFrac), int(iYDelta * p->m_dYMoveFrac));
            rcNew.right += int(iXDelta * p->m_dXSizeFrac);
            rcNew.bottom += int(iYDelta * p->m_dYSizeFrac);
            if (hDefer == NULL)
                hDefer = BeginDeferWindowPos(m_MovingChildren.size());
            UINT uFlags = SWP_NOACTIVATE | SWP_NOOWNERZORDER | SWP_NOZORDER;
            if ((p->m_dXSizeFrac != 0.0) || (p->m_dYSizeFrac != 0.0))
                uFlags |= SWP_NOCOPYBITS;
            DeferWindowPos(hDefer, p->m_hWnd, NULL, rcNew.left, rcNew.top, rcNew.Width(), rcNew.Height(), uFlags);
        }
    }
    if (hDefer != NULL)
        EndDeferWindowPos(hDefer);

    if (m_hGripper != NULL)
        ::ShowWindow(m_hGripper, (nType == SIZE_MAXIMIZED) ? SW_HIDE : SW_SHOW);
}

Since Visual Studio 2015, you can use MFC Dynamic Dialog Layout, but it seems, there is no way to restrict dialog size to minimal size (still only the old way by handling WM_GETMINMAXINFO).

Dynamic layout can be done:

• at design time in resource editor by selecting the control and setting the Moving Type and Sizing Type properties (this emits new AFX_DIALOG_LAYOUT section into .rc file);
• or programatically using the CMFCDynamicLayout class.

Documentation: Dynamic Layout

If your using a dialog template then open the dialog template in the resource editor and set the Style property to Popup and the Border property to Resizing. I'm pretty sure this will do the same as what jussij said and set the WS_POPUP and WS_THICKFRAME styles. To set these dynamically then override the PreCreateWindow function and add the following:

cs.style |= WS_POPUP | WS_THICKFRAME;