I have used P/Invoke to call GetSystemMenu and EnableMenuItem (win32api) to disable the close functionality. However, after minimizing or maximizing my Windows Forms application the button is re-enabled.

Obviously minimizing or maximizing is causing the behavior, but how? I'm not sure where to look to prevent this behavior.

Should I be preventing the maximize and minimize behavior or is there something particularly wrong with the way in which I P/Invoked the calls? Once the application (main form) has loaded, I call the static method from a button click.

class PInvoke
{
    // P/Invoke signatures
    [DllImport("user32.dll")]
    static extern IntPtr GetSystemMenu(IntPtr hWnd, bool bRevert);
    [DllImport("user32.dll")]
    static extern bool EnableMenuItem(IntPtr hMenu, uint uIDEnableItem, uint uEnable);

    // SysCommand (WM_SYSCOMMAND) constant
    internal const UInt32 SC_CLOSE = 0xF060;

    // Constants used with Add/Check/EnableMenuItem
    internal const UInt32 MF_BYCOMMAND = 0x00000000;
    internal const UInt32 MF_ENABLED = 0x00000000;
    internal const UInt32 MF_GRAYED = 0x00000001;
    internal const UInt32 MF_DISABLED = 0x00000002;

    /// <summary>
    /// Sets the state of the Close (X) button and the System Menu close functionality.
    /// </summary>
    /// <param name="window">Window or Form</param>
    /// <param name="bEnabled">Enabled state</param>
    public static void EnableCloseButton(IWin32Window window, bool bEnabled)
    {
        IntPtr hSystemMenu = GetSystemMenu(window.Handle, false);

        EnableMenuItem(hSystemMenu, SC_CLOSE, MF_BYCOMMAND | (bEnabled ? MF_ENABLED : MF_GRAYED));
    }
}