CarBrandViewModel.cs

using System;
using System.Linq;
using System.ComponentModel;
using System.Collections.Generic;
using System.Runtime.CompilerServices;

namespace DrawerViewExample {
    public class CarBrandViewModel : INotifyPropertyChanged  {
        public string BrandName { get; }
        public IReadOnlyList<CarModel> CarModels { get; }

        public event PropertyChangedEventHandler PropertyChanged;

        public CarBrandViewModel(string brandName, IEnumerable<CarModel> carModels) {
            if (String.IsNullOrEmpty(brandName)) {
                this.BrandName = String.Empty;
            }
            else {
                this.BrandName = brandName;
            }
            if (carModels == null) {
                this.CarModels = new List<CarModel>();
            }
            else {
                this.CarModels = carModels.ToList();
            }
        }
        private void RaisePropertyChanged([CallerMemberName] string caller = "") {
            PropertyChangedEventHandler handler = PropertyChanged;
            if (handler != null) {
                handler.Invoke(this, new PropertyChangedEventArgs(caller));
            }
        }
    }
}