做了个小工具，检测当前系统中有没有正在运行的全屏应用，自己不用写，网上也还挺不好找，最后找了一段，有效

代码是这样：

      [StructLayout(LayoutKind.Sequential)]
        public struct RECT
        {
            public int left;
            public int top;
            public int right;
            public int bottom;
        }
        [DllImport("user32.dll", SetLastError = true)]
        public static extern uint GetWindowThreadProcessId(IntPtr hWnd, out uint lpdwProcessId);
        [DllImport("user32.dll")]
        private static extern bool GetWindowRect(HandleRef hWnd, [In, Out] ref RECT rect);
        [DllImport("user32.dll")]
        private static extern IntPtr GetForegroundWindow();
        public static bool IsForegroundFullScreen()
        {
            return IsForegroundFullScreen(null);
        }
        public static bool IsForegroundFullScreen(Screen screen)
        {
            if (screen == null)
            {
                screen = Screen.PrimaryScreen;
            }
            RECT rect = new RECT();
            IntPtr hWnd = (IntPtr)GetForegroundWindow();
            GetWindowRect(new HandleRef(null, hWnd), ref rect);
            if (screen.Bounds.Width == (rect.right - rect.left) && screen.Bounds.Height == (rect.bottom - rect.top))
            {
                Console.WriteLine("Fullscreen");
                return true;
            }
            else
            {
                Console.WriteLine("No Fullscreen");
                return false;
            }
        }

使用：

bool isFullScreen = IsForegroundFullScreen(null);