ToList() method using Single() linq query

I have a problem with ToList() method and basically I'm trying to make a function that will return a var linq query convert to a list and here the the function.

 List<UsersTabPage> GetFirstOne()
 {
    using (MCMDataContext db = new MCMDataContext())
    {
        MCM.User user = new MCM.User();
        var firstone = (from oneUser in db.Users
                        where oneUser.ID == user.ID
                        select oneUser).Single();
        return firstone.ToList();
    }
 }

Here are the librarys that I'm currently using in the application.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
using Microsoft.Win32;
using MahApps.Metro.Controls.Dialogs;
using System.IO;
using System.Data;

If there is any library needed, Please comment below or post an answer. Here is the error.

'MCM.User' does not contain a definition for 'ToList' and no extension method 'ToList' accepting a first argument of type 'MCM.User' could be found (are you missing a using directive or an assembly reference?) If you know any answer please post it below. Thank you.