程序打开一个文本文件，文件中添加内容文字

 #region [ 启动记事本 ]
            System.Diagnostics.Process Proc;
            try
            {
                // 启动记事本
                Proc = new System.Diagnostics.Process();
                Proc.StartInfo.FileName = "notepad.exe";
                Proc.StartInfo.UseShellExecute = false;
                Proc.StartInfo.RedirectStandardInput = true;
                Proc.StartInfo.RedirectStandardOutput = true;
                Proc.Start();
            }
            catch
            {
                Proc = null;
            }
            #endregion


            #region [ 传递数据给记事本 ]
            if (Proc != null)
            {
                // 调用 API, 传递数据
                while (Proc.MainWindowHandle == IntPtr.Zero)
                {
                    Proc.Refresh();
                }
                IntPtr vHandle = FindWindowEx(Proc.MainWindowHandle, IntPtr.Zero, "Edit", null);
                SendMessage(vHandle, WM_SETTEXT, 0, "123123");
            }
            #endregion 

        /// <summary>
        /// 传递消息给记事本
        /// </summary>
        /// <param name="hWnd"></param>
        /// <param name="Msg"></param>
        /// <param name="wParam"></param>
        /// <param name="lParam"></param>
        /// <returns></returns>
        [DllImport("User32.DLL")]
        public static extern int SendMessage(IntPtr hWnd, uint Msg, int wParam, string lParam);

        /// <summary>
        /// 查找句柄
        /// </summary>
        /// <param name="hwndParent"></param>
        /// <param name="hwndChildAfter"></param>
        /// <param name="lpszClass"></param>
        /// <param name="lpszWindow"></param>
        /// <returns></returns>
        [DllImport("User32.DLL")]
        public static extern IntPtr FindWindowEx(IntPtr hwndParent, IntPtr hwndChildAfter, string lpszClass, string lpszWindow);

        /// <summary>
        /// 记事本需要的常量
        /// </summary>
        public const uint WM_SETTEXT = 0x000C;

using System.Runtime.InteropServices;