Cannot implicitly convert type 'System.Collections.Generic.List<string>' to 'System.Collections.Generic.List<Model> Mvc

Your query is selecting a list of the string contains UserNames. And you can not convert List of string to List of FuelerAppointment. You should change to:

 public ActionResult Index()
    {
        var model = new UsersViewModels();
        model.appointmentslist = db.AspNetUsers.Where(
        user => db.FuelerAppointments.Any(f => f.AspNetUserId == user.Id))
       .Select(user => user.FuelerAppointments).ToList();

        return View(model);
    }

You are selecting a list of UserNames, so you will end up with a List<string> and your property is a List<FuelerAppointement>.

Compiler doesn't let you to do this. You declared your list as List<Object> you can't assign it to String. you need to change list if you have username properly in FuelerAppointment

I have tow models same bellow.

 public class FuelerAppointment
{
    public int Id { get; set; }
    public string UserName { get; set; }
    public int AspNetUserId { get; set; }
}

public class AspNetUser
{
    public string UserName { get; set; }
    public int Id { get; set; }
}

and viewModel:

public class UsersViewModels
{
    public List<FuelerAppointment> appointmentslist { get; set; }
}

I think bellow code can help you

 [HttpGet]
    public UsersViewModels Get()
    {
        var model = new UsersViewModels
        {
            appointmentslist = (from user in _dbContext.AspNetUsers
                join fuelerAppointment in _dbContext.FuelerAppointments on user.Id equals fuelerAppointment.AspNetUserId
                select new FuelerAppointment { UserName = user.UserName }).ToList()
        };

        return model;
    }

Result: