//最底层设置
internal class User32
        {
            public const int SE_SHUTDOWN_PRIVILEGE = 0x13;

            [DllImport("user32.dll")]
            public static extern IntPtr FindWindow(string lpClassName, string lpWindowName);

            [DllImport("user32.dll")]
            public static extern IntPtr SetParent(IntPtr hWndChild, IntPtr hWndNewParent);

            [DllImport("user32.dll")]
            public static extern bool SetWindowPos(IntPtr hWnd, int hWndInsertAfter, int X, int Y, int cx,int cy, uint uFlags);
        }

        public MainForm()
        {
            InitializeComponent();
            try
            {
                if (Environment.OSVersion.Version.Major < 6)
                {
                    base.SendToBack();
                    IntPtr hWndNewParent = User32.FindWindow("Progman", null);
                    User32.SetParent(base.Handle, hWndNewParent);
                }
                else
                {
                    User32.SetWindowPos(base.Handle, 1, 0, 0, 0, 0, User32.SE_SHUTDOWN_PRIVILEGE);
                }
            }
            catch (ApplicationException exx)
            {
                MessageBox.Show(this, exx.Message, "Pin to Desktop");
            }
        }

        private void MainForm_Activated(object sender, EventArgs e)
        {
            if (Environment.OSVersion.Version.Major >= 6)
            {
                User32.SetWindowPos(base.Handle, 1, 0, 0, 0, 0, User32.SE_SHUTDOWN_PRIVILEGE);
            }
        }

        private void MainForm_Paint(object sender, PaintEventArgs e)
        {
            if (Environment.OSVersion.Version.Major >= 6)
            {
                User32.SetWindowPos(base.Handle, 1, 0, 0, 0, 0, User32.SE_SHUTDOWN_PRIVILEGE);
            }
        }

//最高层设置
设置form的TopMost属性为True