what is wrong with this method all the time when i call it and give it list and index to get value tell me out of range

public static int GetRoom_SLot_ID(List<int> IndArray, int i)
   {
           int ind = (int)IndArray[i];
           return ind;
   }

 public static List<int> RoomID(int session_id)
    {
        List<int> Room_IDList = new List<int>();

        SqlConnection connecetion = ConnectingDatabase.GetConnection();
        string command = "SELECT Room_ID FROM Room ";
        SqlCommand commn = new SqlCommand(command, connecetion);

        SqlDataReader reader = commn.ExecuteReader();

        while (reader.Read())
        {
            Room_IDList.Add(reader.GetInt32(0));
        }
                connecetion.Close();
               return Room_IDList;
              //this is the method that it return the list >>>and the index is acounter
    }
gettingBlockIDs = GettingValuesIArray.AssignmenBlock_IDs();///getting all IDs in list

/////i call it like that
 block_indx = GettingValuesIArray.GetRoom_SLot_ID(gettingBlockIDs, counter);//this to get value ///inside the list according to the index 

///this is the class with the counter
             counter =0;

        for (int i = 1; i < slots; i++)
        {
            //  studen_number;///i need to check how to get student number
            for (int j = 1; j < rooms; j++)
            {
                ///gettin Assignments IDS
                gettingBlockIDs = GettingValuesIArray.AssignmenBlock_IDs();///getting all IDs in list
                block_indx = GettingValuesIArray.GetRoom_SLot_ID(gettingBlockIDs, counter);
                gettingRooms = GettingValuesIArray.RoomID(1);///getting all rooms IDs in one list
                gettingRoom_ID = GettingValuesIArray.GetRoom_SLot_ID(gettingRooms, i);

                block = new Chromsome(block_indx,gettingRoom_ID);
                room_capacity = block.getRoomCapacity();///getting Room Capacity
                group_id=block.getGroup_ID();

                block2 = new Chromsome(group_id);
                student_Capacity = block2.getGroupCapacity();

                        if (population[i, j] == 0 && student_Capacity <= room_capacity )
                    {

                        population[i, j] = block_indx;
                        counter++;
                        return population; ///i returned the (partial) population in each loop to check for over lapping 

                    }
                    else
                        continue;
                }

            }

            return population;///now i return the final population
        }
    }