List<string> return_list = new List<string>();//当然也可以用数组，不过数组要定义大小
          SqlConnection con = DB.WebConnection();//通过类调用连接上数据库

         con.Open();//打开连接

         SqlCommand com = new SqlCommand();
         com.CommandText = "Select * from Users";
         com.CommandType = CommandType.Text;
         com.Connection = con;

         SqlDataReader reader = com.ExecuteReader();
         while (reader.Read())
         {
             return_list.Add(reader["UserName"].ToString());//循环从库里读取加入到链表
         }