string[] content = System.IO.File.ReadAllText("E:\\test.txt").Split(new char[] { '\r', '\n' }, StringSplitOptions.RemoveEmptyEntries);
            content[1] = "9,f,g,h";
            System.IO.File.WriteAllText("E:\\test.txt", string.Join("\r\n", content));

------解决方案--------------------
探讨

C# code
            string[] content = System.IO.File.ReadAllText("E:\\test.txt").Split(new char[] { '\r', '\n' }, StringSplitOptions.RemoveEmptyEntries);
            content[1] = "9,f,g,h";
            System.IO.File.WriteAllText("E:\\test.txt", string.Join("\r\n", content));


------解决方案--------------------
C# code

            string[] line;
            line = File.ReadAllLines("12.txt",System.Text.Encoding.Default);
            line[1] = "9,f,g,h";
            File.WriteAllLines("12.txt", line);

------解决方案--------------------
用文件流的方式比较好
C# code

//把这个方法直接复制到你的项目中，后面直接调用就行了
 public static void EditFile(int curLine, string newLineValue, string patch)
        {
            FileStream fs = new FileStream(patch, FileMode.Open, FileAccess.Read);
            StreamReader sr = new StreamReader(fs, Encoding.GetEncoding("utf-8"));
            string line = sr.ReadLine();
            StringBuilder sb = new StringBuilder();
            for (int i = 1; line != null; i++)
            {
                sb.Append(line + "\r\n");
                if (i != curLine - 1)
                    line = sr.ReadLine();
                else
                {
                    sr.ReadLine();
                    line = newLineValue;
                }
            }
            sr.Close();
            fs.Close();
            FileStream fs1 = new FileStream(patch, FileMode.Open, FileAccess.Write);
            StreamWriter sw = new StreamWriter(fs1);
            sw.Write(sb.ToString());
            sw.Close();
            fs.Close();
        }

------解决方案--------------------
刚刚9前面多打了一个空格，不好意思
C# code

EditFile(2, "9,f,g,h" ,"c:\\test.txt");//第一个参数是指定修改的行数，第二个参数是你要保存的值,第三个参数是这个文件的路径