<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
    	<ComboBox HorizontalAlignment="Left" Margin="10,21,0,0" VerticalAlignment="Top" Width="120" ItemsSource="{Binding carsList}" SelectedItem="{Binding selectedOne}"/>
    	<Label Content="{Binding selectedOne, Mode=TwoWay}" HorizontalAlignment="Left" Margin="13,66,0,0" VerticalAlignment="Top" Width="117" Height="26"/>
        
    </Grid>
</Window>

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.ComponentModel;
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;

namespace WpfApplication1
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window, INotifyPropertyChanged
    {

        private ObservableCollection<string> _carsList;

        public ObservableCollection<string> carsList
        {
            get { return _carsList; }
            set {
                _carsList = value;
                onPropertyChanged("carsList");
            }
        }

        private string _selectedOne;

        public string selectedOne
        {
            get { return _selectedOne; }
            set 
            {
                _selectedOne = value;
                onPropertyChanged("selectedOne");
            }
        }

        public MainWindow()
        {
            InitializeComponent();

            carsList = new ObservableCollection<string>();
            carsList.Add("TOYOTA");
            carsList.Add("BMW");
            carsList.Add("NISSAN");


            Loaded += MainWindow_Loaded;
        }

        void MainWindow_Loaded(object sender, RoutedEventArgs e)
        {
            this.DataContext = this;
        }

        public event PropertyChangedEventHandler PropertyChanged;
        public void onPropertyChanged(string prop)
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(prop));
            }
        }
    }
}