Data corruption detection in generic app code

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C++

C

Windows

:-D Hi all:
I have a problem here, giving code fragment:

typedef unsigned long HPINDEX; /* Used as a handle */

bool
__ShowWindow(IN HPINDEX hWin)
{
    /*
     * Handle loopup service, converting a handle to
     * a pointer in which we are interested.
     *
     * but all mechnisms exploited here are in user
     * address space, meaning there's a planty of chances
     * for something to go wrong, any better suggestions?
     */
    WND* pWin = (void*)HpGetSlotByIndex(g_hWinSlotMgr, hWin);

    /*
     * Note that the app simply terminates if pWin is non-null
     * but fabricated by the user.
     */
    if (pWin == 0) {
        /*
         * Invalid hWin.
         */
        return (false);
        }

    /*
     * At this phrase, I confused... :(
     * Assume the pWin is valid, and synchronization is
     * provided.
     *
     * Do I have to check data integrity for the entire
     * WND structure, if a dozen of fields are within,
     * too fancy to comprehend.
     *
     * Letting the native API do the hard part of the
     * work I think is the better way, so we check the
     * result of ShowWindow().
     *
     * What if a malicious intention is made managing
     * to find out the structure and fill it with
     * garbage?
     */

     /*
      * We have to check for validity of hwnd field in
      * the very first call which uses it.
      */
    if (!ShowWindow(pWin->hwnd)) {
        /*
         * Report the user about the data corruption. :(
         * Oh man...
         * Is there any way to detect this error at the
         * beginning of __ShowWindow() WITHOUT calling
         * the native API to do that? thus avoid checking
         * the result of the API?
         */
        }
    /*
     * If Checking the result of ShowWindow() is unavoidable,
     * assuming sync is provided by the user, can I take a
     * breath if we survived it, and blindly think that hwnd
     * will always be valid in any subsquent native invocations?
     */
    UpdateWindow(pWin->hwnd);
}

;P Apologize for my pool comments,
what potential risks should be considered by us.
any suggestions are welcome:)

Regards.