FPCM_GET_FLASHVARS
FPCM_PUT_FLASHVARS

struct SFPCGetFlashVars
{    
    // [in, out]
    LPTSTR lpszBuffer;
    DWORD dwBufferSize;

    // [out]
    HRESULT hr;
};


struct SFPCPutFlashVars
{
    // [in]
    LPCTSTR lpszBuffer;

    // [out]
    HRESULT hr;
};

void GetFlashVars(HWND hwndFlashPlayerControl)
{
    SFPCGetFlashVars info;

    info.lpszBuffer = NULL;

    ::SendMessage(hwndFlashPlayerControl, FPCM_GET_FLASHVARS, 0, (LPARAM)&info);

    if FAILED(info.hr)
        // Error
    {
        return;
    }

    info.lpszBuffer = (TCHAR*)LocalAlloc(LPTR, info.dwBufferSize * sizeof(TCHAR));

    ::SendMessage(hwndFlashPlayerControl, FPCM_GET_FLASHVARS, 0, (LPARAM)&info);

    if FAILED(info.hr)
        // Error
    {
        // ...
    }
    else
        // OK
    {
        ::MessageBox(NULL, info.lpszBuffer, _T("Result"), MB_OK);
    }

    LocalFree((HLOCAL)info.lpszBuffer);
}


void PutFlashVars(HWND hwndFlashPlayerControl, LPCTSTR FlashVars)
{
    SFPCPutFlashVars info;

    info.lpszBuffer = FlashVars;

    ::SendMessage(hwndFlashPlayerControl, FPCM_PUT_FLASHVARS, 0, (LPARAM)&info);

    if FAILED(info.hr)
        // Error
    {
        // ...
    }
    else
        // OK
    {
        // ...
    }
}