StreamWriter、StreamReader

        private void button1_Click(object sender, EventArgs e)
        {
            if (textBox1.Text==string.Empty)
            {
                MessageBox.Show("写入内容不能为空");
            }
            else
            {
                saveFileDialog1.Filter = "文本文件（*.txt）|*.txt";//打开保存文件对话框
                if (saveFileDialog1.ShowDialog()==DialogResult.OK)//单机OK按钮
                {
                    StreamWriter sw = new StreamWriter(saveFileDialog1.FileName, true);//创建写入流
                    sw.WriteLine(textBox1.Text);//写入
                    sw.Close();//关闭流
                }
            }
        }
        private void button2_Click(object sender, EventArgs e)
        {
            openFileDialog1.Filter = "文本文件（*.txt）|*.txt";
            if (openFileDialog1.ShowDialog()==DialogResult.OK)
            {
                StreamReader sr = new StreamReader(openFileDialog1.FileName);//创建读出流
                textBox1.Text = sr.ReadToEnd();//读出全部内容
                sr.Close();//关闭流
            }
        }