using System.IO;

        private void button1_Click(object sender, EventArgs e)
        {
            string strFilePath = "";
            OpenFileDialog fd = new OpenFileDialog();
            fd.Filter = "文本文件(*.txt)|*.txt|All files (*.*)|*.*"; //过滤文件类型
            //fd.InitialDirectory = Application.StartupPath + "\\Temp\\";//设定初始文件夹
            //fd.ShowReadOnly = true; //设定文件是否仅仅读
            if (fd.ShowDialog() != DialogResult.OK)
            {
                return;
            }
            strFilePath = fd.FileName;

            if (!File.Exists(strFilePath))
            {
                MessageBox.Show("抱歉。文件不存在。请又一次输入文件名称！
");
                return;
            }

            StreamReader strReader = new StreamReader(new FileStream(strFilePath, FileMode.Open, FileAccess.ReadWrite), Encoding.Default);
            //string strTemp = strReader.ReadLine();
            string strTemp = strReader.ReadToEnd();
            strReader.Close();

            textBox1.Text = strTemp;

        }