am new At C# and web programing. problem in web service error come when i invoke it it is giving me this error System.InvalidOperationException: Unable to generate a temporary class (result=1)...

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Services;

namespace RohitWebService
{
    /// <summary>
    /// Summary description for RohitService
    /// </summary>
    [WebService(Namespace = "http://Rohitshukla.org/")]
    [WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
    [System.ComponentModel.ToolboxItem(false)]
    // To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line.
    // [System.Web.Script.Services.ScriptService]
    public class RohitService : System.Web.Services.WebService
    {

        [WebMethod]
        public string HelloRohit()
        {
            return "code is done by Rohit Shukla";
        }

        [WebMethod]
        public List<String>[] Marks(string sem)
        {

            List<String>[] data =  semester.marks(sem);
            return data;
        }
    }
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Data;
using MySql.Data.MySqlClient;

namespace RohitWebService
{
    public class semester
    {



        public static List<String>[] marks(String mark1)
        {
            int i = 0;
            //Create a list to store the result
            List<string>[] list = new List<string>[4];
            list[0] = new List<string>();
            list[1] = new List<string>();
            list[2] = new List<string>();
            list[3] = new List<string>();




            String mysqlconnection = "server=localhost;;database=rohit_marks;uid=root;password=root;";

            MySqlConnection connection = new MySqlConnection(mysqlconnection);

            MySqlCommand cmd;
            connection.Open();
            try
            {
                //Create Command

                cmd = connection.CreateCommand();
                cmd.CommandText = "select * from marks where sem = @sem ;";
                cmd.Parameters.AddWithValue("@sem", mark1);
                cmd.ExecuteNonQuery();

                //Create a data reader and Execute the command
                MySqlDataReader dataReader = cmd.ExecuteReader();

                //Read the data and store them in the list
                while (dataReader.Read())
                {
                    list[0].Add(dataReader["subject"] + "");
                    list[1].Add(dataReader["sem"] + "");
                    list[2].Add(dataReader["mark_obtained"] + "");

                }

                //close Data Reader
                dataReader.Close();
            }
            catch (MySqlException)
            {
                throw;
            }

            finally
            {
                if (connection.State == ConnectionState.Open)
                {
                    connection.Close();
                }
            }
            return list;

        }
    }
}