Making a WPF system context menu item toggleable

I have the following code, which adds an 'Always on Top' item to the system context menu as displayed on the window chrome. It works correctly, but I'd like it to display a check mark or similar to indicate if it's been toggled on/off.

Any idea how I can do this?

public RibbonShell()
{
    InitializeComponent();

    Loaded += (s,e) =>
                {
                    // Get the Handle for the Forms System Menu
                    var systemMenuHandle = GetSystemMenu(Handle, false);

                    // Create our new System Menu items just before the Close menu item
                    InsertMenu(systemMenuHandle, 5, MfByposition | MfSeparator, 0, string.Empty); // <-- Add a menu seperator
                    InsertMenu(systemMenuHandle, 6, MfByposition, SettingsSysMenuId, "Always on Top");

                    // Attach our WindowCommandHandler handler to this Window
                    var source = HwndSource.FromHwnd(Handle);
                    source.AddHook(WindowCommandHandler);
                };
}

#region Win32 API Stuff

// Define the Win32 API methods we are going to use
[DllImport("user32.dll")]
private static extern IntPtr GetSystemMenu(IntPtr hWnd, bool bRevert);

[DllImport("user32.dll")]
private static extern bool InsertMenu(IntPtr hMenu, Int32 wPosition, Int32 wFlags, Int32 wIDNewItem, string lpNewItem);

/// Define our Constants we will use
private const int WmSyscommand = 0x112;
private const int MfSeparator = 0x800;
private const int MfByposition = 0x400;

#endregion

// The constants we'll use to identify our custom system menu items
private const int SettingsSysMenuI开发者_如何学运维d = 1000;

/// <summary>
/// This is the Win32 Interop Handle for this Window
/// </summary>
public IntPtr Handle
{
    get { return new WindowInteropHelper(this).Handle; }
}

private IntPtr WindowCommandHandler(IntPtr hwnd, int msg, IntPtr wParam, IntPtr lParam, ref bool handled)
{
    // Check if a System Command has been executed
    if (msg == WmSyscommand && wParam.ToInt32() == SettingsSysMenuId)
    {
        Topmost = !Topmost;
        handled = true;
    }

    return IntPtr.Zero;
}

You need to call CheckMenuItem whenever you change Topmost. See the CheckMenuItem documentaton for details. Here's the P/Invoke signature and constants you'll need:

[DllImport("user32.dll")] 
private static extern bool CheckMenuItem(IntPtr hMenu, Int32 uIDCheckItem, Int32 uCheck); 

private const int MfChecked = 8;
private const int MfUnchecked = 0;

Now to check the item, just:

CheckMenuItem(systemMenuHandle, SettingsSysMenuId, MfChecked);

and to uncheck:

CheckMenuItem(systemMenuHandle, SettingsSysMenuId, MfUnchecked);