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public class WindowEx : Window //...
{
        //hwnd окна
        public IntPtr Handle { get; set; }

        private const int GWL_EXSTYLE = (-20);
        private const uint WS_EX_TOPMOST = 0x00000008;

        public void SetTopmost()
        {
            SetTopmost(Handle);
        }

        public static void SetTopmost(IntPtr hWnd)
        {
            SetWindowLongPtr(hWnd, GWL_EXSTYLE, (IntPtr)((ulong)GetWindowLongPtr(hWnd, GWL_EXSTYLE) | WS_EX_TOPMOST));
        }

        public void UnSetTopmost()
        {
            UnSetTopmost(Handle);
        }

        public static void UnSetTopmost(IntPtr hWnd)
        {
            SetWindowLongPtr(hWnd, GWL_EXSTYLE, (IntPtr)((ulong)GetWindowLongPtr(hWnd, GWL_EXSTYLE) & ~WS_EX_TOPMOST));
        }

        private bool _topmost;
        public new bool Topmost
        {
            set
            {
                if (value)
                {
                    SetTopmost();
                }
                else
                {
                    UnSetTopmost();
                }
                _topmost = value;
            }
            get { return _topmost; }
        }
}