MainWindow.xaml

<Window x:Class="WpfApplication6.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>
        <StackPanel>
            <ComboBox ItemsSource="{Binding C}" SelectedValue="{Binding S}"/>
            <Button Click="ButtonBase_OnClick">B</Button>
            <Button Click="ButtonBase_OnClick2">B2</Button>
        </StackPanel>
    </Grid>
</Window>

MainWindow.xaml.cs

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

namespace WpfApplication6
{
    public partial class MainWindow : Window, INotifyPropertyChanged
    {
        private int s;
        private IEnumerable<int> c = new ObservableCollection<int>();

        public MainWindow()
        {
            InitializeComponent();
            this.DataContext = this;
        }

        public int S
        {
            get { return s; }
            set
            { 
                s = value;
                if (this.PropertyChanged != null)
                {
                    this.PropertyChanged(this, new PropertyChangedEventArgs("S"));
                }
            }
        }

        public IEnumerable<int> C
        {
            get { return c; }
            set
            {
                c = value;
                if (this.PropertyChanged != null)
                {
                    this.PropertyChanged(this, new PropertyChangedEventArgs("C"));
                }
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;

        private void ButtonBase_OnClick(object sender, RoutedEventArgs e)
        {
            S = 1;
        }

        private void ButtonBase_OnClick2(object sender, RoutedEventArgs e)
        {
            C = new ObservableCollection<int> { 2, 3 };
            C = new ObservableCollection<int> { 1, 2 };
        }
    }
}