打开文件

    public class MyFile
    {
        /// <summary>
        /// 定义属性获取文件路径
        /// </summary>
        public string PathFileName { get; set; } 

      public  MyFile()
        {

        }
        /// <summary>
        /// 构造函数初始化值
        /// </summary>
        /// <param name="pathFileName">文件路径</param>
       public MyFile(string pathFileName)
        {
            this.PathFileName = pathFileName;
        }

        public void OpenFile() //打开文件的进程方法
        {
            ProcessStartInfo pro = new ProcessStartInfo(PathFileName);//启动进程获取文件路径
            Process pr = new Process();//开启进程
            pr.StartInfo = pro;//赋值
            pr.Start();//打开

        }
    }
    class TxtFile : MyFile
    {
        public TxtFile(string pathFileName) : base(pathFileName)
        {

        }
    }
}

 private void button1_Click(object sender, EventArgs e)
        {
            string fileName;
            OpenFileDialog op = new OpenFileDialog();//打开对话框
            op.ShowDialog();
            fileName = op.FileName;//获取文件名
            textBox1.Text = fileName;
            string fileExtension=   Path.GetExtension(fileName);//获取扩展名

            //打开
            MyFile my = new MyFile(fileName);
            try
            {
                my = OpenFileStr(fileExtension, fileName);
                my.OpenFile();
            }
            catch
            {
                MessageBox.Show("输入的格式有误");
                textBox1.Text = null;
            }     
            
        }
        /// <summary>
        /// 打开文件的方法
        /// </summary>
        /// <param name="fileExtension">扩展名</param>
        /// <param name="fileName">文件路径</param>
        /// <returns></returns>
        public MyFile OpenFileStr(string fileExtension,string fileName)//
        {
            MyFile my = null;
            switch (fileExtension)
            {
                case ".txt":
                    my = new TxtFile(fileName);
                    break;
            }
            return my;
        }