组合两个字符串的问题
我想将连个字符串组成起来，但是为什么其中一个字符串会提示为赋值字段？求指教？

C# code

protected string GetFormCode()        {            int maxValue = 0;            string classname = ddlclass.Text.Trim();            string code;            string code1;            // using (LXS.DAL.deviceinfo.LXS db = new LXS.DAL.deviceinfo.LXS())            //DataTable dt=new DataTable ();            {                SqlConnection conn = new SqlConnection("Data Source=(local);Database=LOGINDB;Uid=sa;Pwd=sa123456");                SqlCommand cmd;                SqlCommand cmd1;                cmd = conn.CreateCommand();                cmd1 = conn.CreateCommand();                cmd.CommandText = "select max (a.deviceid)from (select deviceid from deviceinfo where classno=(select classno from class where classname='" + classname + "')) a";                cmd1.CommandText = "select classno from deviceinfo where classno=(select classno from class where classname='" + classname + "') group by classno";                conn.Open();                SqlDataReader Reader = cmd.ExecuteReader();                                while (Reader.Read())                {                    maxValue = Convert.ToInt32(Reader[0].ToString());                }                                code = Convert.ToString(maxValue + 1);                Reader.Close();                conn.Close();                conn.Open();                SqlDataReader Reader1 = cmd1.ExecuteReader();                while (Reader1.Read())                {                   code1 = Convert.ToString(Reader1[0].ToString());                  }              //  code1 = Convert.ToString(code1);                Reader.Close();                conn.Close();            //    code = "mm" + code;（如果改成此句，则可以操作）                code = code1+code;                                return code;}