private static DialogResult ShowCore(IWin32Window owner, string text, string caption,   
                                             MessageBoxButtons buttons, MessageBoxIcon icon, MessageBoxDefaultButton defaultButton,
                                             MessageBoxOptions options, bool showHelp) {
       
            if (!ClientUtils.IsEnumValid(buttons, (int)buttons, (int)MessageBoxButtons.OK, (int)MessageBoxButtons.RetryCancel)){
               throw new InvalidEnumArgumentException("buttons", (int)buttons, typeof(MessageBoxButtons));
            }
 
            // valid values are 0x0 0x10 0x20 0x30 0x40, chop off the last 4 bits and check that it's between 0 and 4.
            if (!WindowsFormsUtils.EnumValidator.IsEnumWithinShiftedRange(icon, /*numBitsToShift*/4, /*min*/0x0,/*max*/0x4)) {
                throw new InvalidEnumArgumentException("icon", (int)icon, typeof(MessageBoxIcon));
            }
            // valid values are 0x0 0x100, 0x200, chop off the last 8 bits and check that it's between 0 and 2.
            if (!WindowsFormsUtils.EnumValidator.IsEnumWithinShiftedRange(defaultButton, /*numBitsToShift*/8, /*min*/0x0,/*max*/0x2)) {
                throw new InvalidEnumArgumentException("defaultButton", (int)defaultButton, typeof(DialogResult));
            }
            
            // options intentionally not verified because we don't expose all the options Win32 supports.
 
            if (!SystemInformation.UserInteractive && (options & (MessageBoxOptions.ServiceNotification | MessageBoxOptions.DefaultDesktopOnly)) == 0) {
                throw new InvalidOperationException(SR.GetString(SR.CantShowModalOnNonInteractive));
            }
            if (owner != null && (options & (MessageBoxOptions.ServiceNotification | MessageBoxOptions.DefaultDesktopOnly)) != 0) {
                throw new ArgumentException(SR.GetString(SR.CantShowMBServiceWithOwner), "options");
            }
            if (showHelp && (options & (MessageBoxOptions.ServiceNotification | MessageBoxOptions.DefaultDesktopOnly)) != 0) {
                throw new ArgumentException(SR.GetString(SR.CantShowMBServiceWithHelp), "options");
            }
 
            // demand if not safe known options.
            if ((options & ~(MessageBoxOptions.RightAlign | MessageBoxOptions.RtlReading)) != 0) {
                // See DDB#163043.
                IntSecurity.UnmanagedCode.Demand();
            }
 
            IntSecurity.SafeSubWindows.Demand();
 
            int style = (showHelp) ? HELP_BUTTON : 0; 
            style |= (int) buttons | (int) icon | (int) defaultButton | (int) options;
 
            IntPtr handle = IntPtr.Zero;
            if (showHelp || ((options & (MessageBoxOptions.ServiceNotification | MessageBoxOptions.DefaultDesktopOnly)) == 0)) {
                if (owner == null) {
                    handle = UnsafeNativeMethods.GetActiveWindow();
                }
                else {
                    handle = Control.GetSafeHandle(owner);
                }
            }
 
            IntPtr userCookie = IntPtr.Zero;
 
            if (Application.UseVisualStyles) {
                // CLR4.0 or later, shell32.dll needs to be loaded explicitly.
                if (UnsafeNativeMethods.GetModuleHandle(ExternDll.Shell32) == IntPtr.Zero) {
                    if (UnsafeNativeMethods.LoadLibrary(ExternDll.Shell32) == IntPtr.Zero) {
                        int lastWin32Error = Marshal.GetLastWin32Error();
                        throw new Win32Exception(lastWin32Error, SR.GetString(SR.LoadDLLError, ExternDll.Shell32));
                    }
                }
 
                // Activate theming scope to get theming for controls at design time and when hosted in browser.
                // NOTE: If a theming context is already active, this call is very fast, so shouldn't be a perf issue.
                userCookie = >>UnsafeNativeMethods.ThemingScope.Activate();<<<
            }
 
            Application.BeginModalMessageLoop();
            DialogResult result;
            try {
                result = Win32ToDialogResult(SafeNativeMethods.MessageBox(new HandleRef(owner, handle), text, caption, style));
            }
            finally {
                Application.EndModalMessageLoop();
                UnsafeNativeMethods.ThemingScope.Deactivate(userCookie);
            }
 
            // Right after the dialog box is closed, Windows sends WM_SETFOCUS back to the previously active control 
            // but since we have disabled this thread main window the message is lost. So we have to send it again after
            // we enable the main window.
            //
            UnsafeNativeMethods.SendMessage(new HandleRef(owner, handle), NativeMethods.WM_SETFOCUS, 0, 0);
            return result;
        }