MainWindowViewModel.cs

using System;
using System.Runtime.CompilerServices;
using System.Windows.Input;

namespace CodeExamples.Wpf.Interactivity
{
    public class MainWindowViewModel : ViewModelBase
    {
        #region Fields and properties
		
        private string SelectedItem;

        public string[] AvailableComboBoxValues
        {
            get { return new[] { "E", "W" }; }
        }

        public string SelectedComboBoxItem
        {
            get { return SelectedItem; }
            set
            {
                SelectedItem = value;
                RaisePropertyChanged();
            }
        }
        #endregion

        #region Constructors

        public MainWindowViewModel() : base() { }
        #endregion

        #region ViewModelBase methods overriding

        protected override void RaisePropertyChanged([CallerMemberName] string propertyName = "")
        {
            base.RaisePropertyChanged(propertyName);

            if (string.Equals(propertyName, nameof(SelectedComboBoxItem), StringComparison.Ordinal))
            {
                // do something you need
            }
        }
        #endregion
    }
}