MainViewModel.cs

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

namespace DrawerViewExample {
    public class MainViewModel : INotifyPropertyChanged {
        private static readonly IReadOnlyList<CarModel> allCarModels = new List<CarModel> {
            new CarModel("Mercedes-Benz", "SL500 Roadster"),
            new CarModel("Mercedes-Benz", "CLK55 AMG Cabriolet"),
            new CarModel("Mercedes-Benz", "C230 Kompressor Sport Coupe"),
            new CarModel("BMW", "530i"),
            new CarModel("Rolls-Royce", "Corniche"),
            new CarModel("Jaguar", "S-Type 3.0"),
            new CarModel("Cadillac", "Seville"),
            new CarModel("Cadillac", "DeVille"),
            new CarModel("Lexus", "LS430"),
            new CarModel("Lexus", "GS430"),
            new CarModel("Ford", "Ranger FX-4"),
            new CarModel("Dodge", "RAM 1500"),
            new CarModel("GMC", "Siera Quadrasteer"),
            new CarModel("Nissan", "Crew Cab SE"),
            new CarModel("Toyota", "Tacoma S-Runner"),
        };

        public IReadOnlyList<CarBrandViewModel> CarModelsByBrand { get; }

        public event PropertyChangedEventHandler PropertyChanged;

        public MainViewModel() {
            List<CarBrandViewModel> carBrandViewModels = new List<CarBrandViewModel>();
            carBrandViewModels.Add(new CarBrandViewModel("All", allCarModels));

            IEnumerable<IGrouping<string, CarModel>> groupedCarModels = 
                                                        allCarModels.GroupBy(v => v.BrandName);
            foreach (IGrouping<string, CarModel> carModelGroup in groupedCarModels) {
                carBrandViewModels.Add(new CarBrandViewModel(carModelGroup.Key, carModelGroup));
            }
            CarModelsByBrand = carBrandViewModels;
        }

        private void RaisePropertyChanged([CallerMemberName] string caller = "") {
            PropertyChangedEventHandler handler = PropertyChanged;
            if (handler != null) {
                handler.Invoke(this, new PropertyChangedEventArgs(caller));
            }
        }
    }
}