I wrote a blog before. It's a way to draw a rectangle, but today's method is better than the previous one. It's convenient for me and my friends......
Direct code:
Write the following code in the header file

    protected:
	HICON m_hIcon;
	CPoint m_ptbegin;
	CPoint m_ptEnd;
	BOOL m_blBtnDown;
	CDC m_dcMemory;

In the implementation file:

void CModifyRectDlg::OnLButtonDown(UINT nFlags, CPoint point)
{
	// TODO: add message handler code and / or call defaults here
	//GetCursorPos(&point);
	
    CWnd *pWnd = GetDlgItem(IDC_STATIC);
	CDC *pDC = pWnd->GetDC();
	CRect rcClient1;
	GetCursorPos(&point);
	pWnd->ScreenToClient(&point);
	pWnd->GetClientRect(&rcClient1);
	if(point.x> rcClient1.left&&point.x <rcClient1.right&&point.y <rcClient1.bottom&&point.y> rcClient1.top)//Limit click location
	{
	//ClipCursor(&rcClient);
	m_blBtnDown = TRUE;
	
	
	m_ptbegin = point;
   // pWnd->GetWindowRect(&rcClient);
	//ScreenToClient(&point);
	
	CBitmap oBitmap;
	oBitmap.CreateCompatibleBitmap(pDC, rcClient1.Width(), rcClient1.Height());//Create an object that is the same as the original
 
	if (m_dcMemory.m_hDC != NULL)
	{
		m_dcMemory.DeleteDC();
	}
	m_dcMemory.CreateCompatibleDC(pDC);//First, set up the data in this DC, and then complete the refresh of this window in the DC copied to the window
	m_dcMemory.SelectObject(&oBitmap);
	m_dcMemory.BitBlt(0, 0, rcClient1.Width(), rcClient1.Height(), pDC, 0, 0, SRCCOPY);//Transfer the device of one memory block to another
 
	oBitmap.DeleteObject();
 
	ReleaseDC(pDC);

	}
	CDialog::OnLButtonDown(nFlags, point);
}

This is a double dc method. CreateCompatibleBitmap, CreateCompatibleDC copies a dc rectangle for drawing, and BitBlt transfers the device of one memory block to another memory block for display

void CModifyRectDlg::OnMouseMove(UINT nFlags, CPoint point)
{
	// TODO: add message handler code and / or call defaults here
	
	if (m_blBtnDown==TRUE)
	{
		CWnd *pWnd = GetDlgItem(IDC_STATIC);
	    CDC *pDC = pWnd->GetDC();	   
		CRect rcClient;
		pWnd->GetClientRect(&rcClient);
		GetCursorPos(&point);
	    pWnd->ScreenToClient(&point);
		if(point.x> rcClient.left&&point.x <rcClient.right&&point.y <rcClient.bottom&&point.y> rcClient.top)
		{		
		pDC->BitBlt(0, 0, rcClient.Width(), rcClient.Height(), &m_dcMemory, 0, 0, SRCCOPY);
	    pDC->SetROP2(R2_NOTXORPEN); //Transparent rectangle
		CRect rc(m_ptbegin, point);
		rc.NormalizeRect();
		pDC->Rectangle(&rc);
		//ScreenToClient(&rc);//
		ReleaseDC(pDC);
		}
	}
	CDialog::OnMouseMove(nFlags, point);
}

void CModifyRectDlg::OnLButtonUp(UINT nFlags, CPoint point)
{
	// TODO: add message handler code and / or call defaults here
	m_blBtnDown = FALSE;
	CWnd *pWnd = GetDlgItem(IDC_STATIC);
    CDC *pDC = pWnd->GetDC();
	pDC->SetROP2(R2_NOTXORPEN);
	CPen pen(PS_SOLID,2,RGB(0,255,0));//Draw a colored rectangle with a brush
    CPen* pOldPen;
    pOldPen=pDC->SelectObject (&pen);
		CRect rcClient;
		pWnd->GetClientRect(&rcClient);
		GetCursorPos(&point);
	    pWnd->ScreenToClient(&point);
	if(point.x> rcClient.left&&point.x <rcClient.right&&point.y <rcClient.bottom&&point.y> rcClient.top)
	{
	m_ptEnd = point;
	CRect rc(m_ptbegin, m_ptEnd);
	rc.NormalizeRect();
	pDC->Rectangle(&rc);
	m_dcMemory.DeleteDC();//Remove device context
	}
	//Invalidate();
	CDialog::OnLButtonUp(nFlags, point);
}

Results: