[DllImport("user32.dll")]
        [return: MarshalAs(UnmanagedType.Bool)]
        public static extern bool GetGUIThreadInfo(uint idThread, ref GUITHREADINFO lpgui);
        [DllImport("user32.dll")]
        public static extern IntPtr GetForegroundWindow();
        [DllImport("user32.dll", SetLastError = true)]
        static extern uint GetWindowThreadProcessId(IntPtr hWnd, out uint lpdwProcessId);

        public struct GUITHREADINFO
        {
            public int cbSize;
            public int flags;
            public int hwndActive;
            public int hwndFocus;
            public int hwndCapture;
            public int hwndMenuOwner;
            public int hwndMoveSize;
            public int hwndCaret;
            public System.Drawing.Rectangle rcCaret;
        }

//以下写在某个方法里

       IntPtr hWnd = GetForegroundWindow();

            List<IntPtr> list = GetChildWindows(hWnd);

//添加for循环
            uint processId;
            uint threadid = GetWindowThreadProcessId(hWnd, out processId);

            GUITHREADINFO lpgui = new GUITHREADINFO();
            lpgui.cbSize = Marshal.SizeOf(lpgui);

            if (GetGUIThreadInfo(threadid, ref lpgui))
            {

                if (lpgui.hwndCaret != 0)
                {

                    //获取文本内容

                    StringBuilder str = new StringBuilder();

                    GetWindowText(list[i], str, 800);
                    this.txtContext.Text = str.ToString();
                    break;
                }
            }

 public delegate bool EnumWindowProc(IntPtr hWnd, IntPtr parameter);
        [DllImport("user32")]
        [return: MarshalAs(UnmanagedType.Bool)]
        public static extern bool EnumChildWindows(IntPtr window, EnumWindowProc callback, IntPtr i);
        public static List<IntPtr> GetChildWindows(IntPtr parent)
        {
            List<IntPtr> result = new List<IntPtr>();
            GCHandle listHandle = GCHandle.Alloc(result);
            try
            {
                EnumWindowProc childProc = new EnumWindowProc(EnumWindow);
                EnumChildWindows(parent, childProc, GCHandle.ToIntPtr(listHandle));
            }
            finally
            {
                if (listHandle.IsAllocated)
                    listHandle.Free();
            } return result;
        }

        private static bool EnumWindow(IntPtr handle, IntPtr pointer)
        {
            GCHandle gch = GCHandle.FromIntPtr(pointer);
            List<IntPtr> list = gch.Target as List<IntPtr>;
            if (list == null)
                throw new InvalidCastException("GCHandle Target could not be cast as List<IntPtr>");
            list.Add(handle);
            return true;
        }