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A Better CenterWindow() Function

, 3 Oct 2000
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This is a good replacement for CWnd::CenterWindow() that works.
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    Sample Image - CenterAnywhere1.gif
    Figure 1. The main CenterSample sample program screen.

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    Introduction

    Centering windows on the screen is something which you can normally do with the CWnd::CenterWindow() function in MFC. CenterWindow() takes a pointer to a CWnd as its argument, and supposedly the function will center the window on which it's called against the window to which you pass it a pointer:

    pDlg->CenterWindow(AfxGetMainWnd()); // Centers pDlg against the main window?
    Listing 1. Demonstrating a use of CWnd::CenterWindow() to center a dialog.

    However, a question posed to the MFC Mailing List recently asked, "I have a dialog-based program, where the user can click a button and have a sub-dialog pop up. If I call CWnd::CenterWindow() in the sub-dialog's OnInitDialog() handler, the dialog will always be centered in the center of the screen, not centered with respect to the main dialog. How do I do this?"

    So I came up with a "Brute Force" centering function which actually works better than CWnd::CenterWindow(). It's called CSubDialog::CenterWindowOnOwner(), and I added to my sample program's sub-dialog's class, CSubDialog:

    void CSubDialog::CenterWindowOnOwner(CWnd* pWndToCenterOn)
    {
        // Get the client rectangle of the window on which we want to center
        // Make sure the pointer is not NULL first
    
        if (pWndToCenterOn == NULL)
            return;
     
        CRect rectToCenterOn;
        pWndToCenterOn->GetWindowRect(&rectToCenterOn);
    
        // Get this window's area
        CRect rectSubDialog;
        GetWindowRect(&rectSubDialog);
    
        // Now rectWndToCenterOn contains the screen rectangle of the window 
        // pointed to by pWndToCenterOn.  Next, we apply the same centering 
        // algorithm as does CenterWindow()
    
        // find the upper left of where we should center to
    
        int xLeft = (rectToCenterOn.left + rectToCenterOn.right) / 2 - 
            rectSubDialog.Width() / 2;
        int yTop = (rectToCenterOn.top + rectToCenterOn.bottom) / 2 - 
            rectSubDialog.Height() / 2;
    
        // Move the window to the correct coordinates with SetWindowPos()
        SetWindowPos(NULL, xLeft, yTop, -1, -1,
            SWP_NOSIZE | SWP_NOZORDER | SWP_NOACTIVATE);
    }
    Listing 2. Our brute-force CenterWindowOnOwner() function.

    Then I added code to CSubDialog::OnInitDialog() to center it with respect to the main dialog, which is the main window of the application:

    BOOL CSubDialog::OnInitDialog()
    {
        CDialog::OnInitDialog();
    
        ...
    
        CWnd* pMainWnd = AfxGetMainWnd();
        CenterWindowOnOwner(pMainWnd);
    
        return TRUE;
    }
    Listing 3.How to call CenterWindowOnOwner().

    And voila! The sub-dialog will always center itself on the main dialog (or main application window), no matter where on the screen that window is placed.

    License

    This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

    A list of licenses authors might use can be found here

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    About the Author

    Brian C Hart
    Software Developer (Senior) Corrugated Technologies, Inc.
    United States United States
    From Fridley, Minnesota and I like computer programming! When I got started, I was working mostly with Windows GUI programming in C/C++. Then later on I worked with COM/DCOM for a school internship. I used COM/DCOM to write an ad hoc cluster server and job-running environment for a cluster of 24 Windows-based high-end visualization workstations. I moved on to C# and have been working in C# and Windows Forms ever since. I have yet to embrace Silverlight Smile | :)
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    Comments and Discussions

     
    GeneralTruely multi-monitor aware CenterWindow code Pinmemberryltsov20-Jun-08 1:25 
    Generalcentering data inside a CMDIChildWnd PinsussAnonymous18-Sep-04 21:33 
    GeneralMultiple-monitor support PinmemberLeo Davidson20-Jun-01 6:25 
    GeneralRe: Multiple-monitor support PinmemberLeo Davidson20-Jun-01 6:31 
    GeneralRe: Multiple-monitor support Pinmemberandi payn1-Oct-01 13:15 
    GeneralRe: Multiple-monitor support Pinmember_Stilgar_1-Mar-07 10:14 
    Generalbulletproof CenterWindow function for multi-monitors PinmemberAlfie Weinie13-Feb-01 14:38 
    GeneralRe: bulletproof CenterWindow function for multi-monitors Pinmemberyelena22s15-Aug-05 14:58 
    GeneralSuggestion PinsussDominic I. Holmes9-Oct-00 8:40 
    GeneralCenter Form on CFormView PinsussGfw7-Oct-00 12:41 
    GeneralRe: Center Form on CFormView PinmemberVirender Sandhu14-Nov-02 17:39 

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