能给我描述这段代码，每一行，每一个字符都非常清楚，谢谢大家

void CEx_DlgDrawDlg::DoDrawCtrl()

{

 CWnd *pWnd = GetDlgItem( IDC_DRAW );

 CDC* pDC = pWnd->GetDC(); 

 CRect rcClient;

 pWnd->GetClientRect( rcClient ); // 获取控件客户区大小

 UpdateWindow(); // 告诉对话框，控件已更新过

 CBrush brush(HS_CROSS,RGB(0,0,255));

 CBrush *oldBrush = pDC->SelectObject( &brush );

 pDC->Rectangle( rcClient );

 pDC->SelectObject( oldBrush );

}

void CEx_DlgDrawDlg::DoDrawCtrl()

{

 CWnd *pWnd = GetDlgItem( IDC_DRAW ); //获取控件对象

 CDC* pDC = pWnd->GetDC(); //根据对象获取绘图上下文的设备HDC

 CRect rcClient;

 pWnd->GetClientRect( rcClient ); // 获取控件客户区大小

 UpdateWindow(); // 告诉对话框，控件已更新过

 CBrush brush(HS_CROSS,RGB(0,0,255));//创建画刷

 CBrush *oldBrush = pDC->SelectObject( &brush );//把创建的画刷选入到绘图设备中,并保存原有的画刷

 pDC->Rectangle( rcClient );//绘制矩形

 pDC->SelectObject( oldBrush );//将原来的画刷还回给绘图设备

}