Switch the state of close button on WinForms.

FromExtensions.cs

using System;
using System.Runtime.InteropServices;
using System.Windows.Forms;

namespace TestForm
{
    public static class FormExtensions
    {
        [DllImport("USER32.DLL")]
        private static extern IntPtr GetSystemMenu(IntPtr hWnd, bool bRevert);
        [DllImport("USER32.DLL")]
        private static extern bool EnableMenuItem(IntPtr hMenu, uint uIDEnableItem, uint uEnable);
        
        private const uint SC_CLOSE = 0x0000F060;
        private const uint MF_BYCOMMAND = 0x00000000;
        private const uint MF_ENABLED = 0x00000000;
        private const uint MF_GRAYED = 0x00000001;
        private const uint MF_DISABLED = 0x00000002;

        public static void EnableCloseButton(this Form form, bool enable)
        {
            IntPtr hMenu = GetSystemMenu(form.Handle, false);
            uint command = enable ? MF_ENABLED : (MF_GRAYED | MF_DISABLED);
            EnableMenuItem(hMenu, SC_CLOSE, MF_BYCOMMAND | command);
        }
    }
}