“system_program.system.getMAX(string, string, string, string)”: 并非所有的代码路径都返回值 无法返回值

Question

刚学C＃不久，写了一段代码，运行时出现错误，提示并非所有的代码都有返回值，请大家帮我看看下面这段代码

        public static string getMAX(string Id,string DataTable,string Condition,string Conditionvalue) 
        {
            string Maxsql = "select max(" + Id + ")from" + DataTable + "where" + Condition + "=" + Conditionvalue + "";
            SqlConnection con = new SqlConnection(SQLconnection);
            con.Open();
            SqlCommand cmdText = new SqlCommand();
            cmdText.Connection = con;
            cmdText.CommandText = Maxsql;
            SqlDataReader views = cmdText.ExecuteReader();
            int Max = 0;
            while (views.Read())
            {
                if (views[0].ToString() == "")
                {
                     return "0001";
                }
                else
                {
                    Max = Convert.ToInt32(views[0].ToString());
                    ++Max;
                    string MaxNum = string.Format("0:0000}", Max);
                    return MaxNum;
                }
            }
        }