判断是否有全屏程序正在运行（C#）

注册一个AppBar（什么是AppBar？Using Application Desktop Toolbars），通过SHAppBarMessage向系统注册AppBar，这样，当有程序全屏运行时系统会向我们的程序发送消息，在窗体WndProc中处理即可。

声明要使用到的API和常量：

 public class APIWrapper
        {
            [DllImport("SHELL32", CallingConvention = CallingConvention.StdCall)]
            public static extern uint SHAppBarMessage(int dwMessage, ref  APPBARDATA pData);
            [DllImport("User32.dll", CharSet = CharSet.Auto)]
            public static extern int RegisterWindowMessage(string msg);
        }
        [StructLayout(LayoutKind.Sequential)]
        public struct RECT
        {
            public int left;
            public int top;
            public int right;
            public int bottom;
        }
        [StructLayout(LayoutKind.Sequential)]
        public struct APPBARDATA
        {
            public int cbSize;
            public IntPtr hWnd;
            public int uCallbackMessage;
            public int uEdge;
            public RECT rc;
            public IntPtr lParam;
        }
        public enum ABMsg : int
        {
            ABM_NEW = 0,
            ABM_REMOVE,
            ABM_QUERYPOS,
            ABM_SETPOS,
            ABM_GETSTATE,
            ABM_GETTASKBARPOS,
            ABM_ACTIVATE,
            ABM_GETAUTOHIDEBAR,
            ABM_SETAUTOHIDEBAR,
            ABM_WINDOWPOSCHANGED,
            ABM_SETSTATE
        }
        public enum ABNotify : int
        {
            ABN_STATECHANGE = 0,
            ABN_POSCHANGED,
            ABN_FULLSCREENAPP,
            ABN_WINDOWARRANGE
        }
        public enum ABEdge : int
        {
            ABE_LEFT = 0,
            ABE_TOP,
            ABE_RIGHT,
            ABE_BOTTOM
        }
        int uCallBackMsg;
        bool RunningFullScreenApp;
        private void RegisterAppBar(bool registered)
        {
            APPBARDATA abd = new APPBARDATA();
            abd.cbSize = Marshal.SizeOf(abd);
            abd.hWnd = this.Handle;
            if (!registered)
            {
                //register  
                uCallBackMsg = APIWrapper.RegisterWindowMessage("APPBARMSG_CSDN_HELPER");
                abd.uCallbackMessage = uCallBackMsg;
                uint ret = APIWrapper.SHAppBarMessage((int)ABMsg.ABM_NEW, ref  abd);
            }
            else
            {
                APIWrapper.SHAppBarMessage((int)ABMsg.ABM_REMOVE, ref  abd);
            }
        }
        protected override void WndProc(ref  System.Windows.Forms.Message m)
        {
            if (m.Msg == uCallBackMsg)
            {
                switch (m.WParam.ToInt32())
                {
                    case (int)ABNotify.ABN_FULLSCREENAPP:
                        {
                            if ((int)m.LParam == 1)
                                this.RunningFullScreenApp = true;
                            else
                                this.RunningFullScreenApp = false;
                            break;
                        }
                    default:
                        break;
                }
            }
            base.WndProc(ref  m);
        }

 private void Form1_Load(object sender, EventArgs e)
        {
            this.RegisterAppBar(false);
        }

在程序退出时，不要忘了Unregister我们注册的AppBar：

private void Form1_FormClosed(object sender, FormClosedEventArgs e)
        {
            this.RegisterAppBar(true);
        }