gistfile1.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;
using System.Text;
using System.ComponentModel;
using WpfApplication1.Models;

namespace WpfApplication1.ViewModels
{
    public class PersonEditViewModel : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged = (s, e) => { };

        private Person _person = new Person();

        public string FirstName {
            get { return _person.FirstName; }
            set {
                _person.FirstName = value;
                PropertyChanged.Notify(() => FirstName);
                PropertyChanged.Notify(() => DisplayName);
            }
        }

        public string MiddleName {
            get { return _person.MiddleName; }
            set {
                _person.MiddleName = value;
                PropertyChanged.Notify(() => MiddleName);
                PropertyChanged.Notify(() => DisplayName);
            }
        }

        public string LastName {
            get { return _person.LastName; }
            set {
                _person.LastName = value;
                PropertyChanged.Notify(() => LastName);
                PropertyChanged.Notify(() => DisplayName);
            }
        }

        public string DisplayName {
            get { return _person.DisplayName; }
        }
        
    }
}

gistfile2.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Linq.Expressions;
using System.Reflection;
using System.ComponentModel;

namespace WpfApplication1.ViewModels
{
    public static class Helpers
    {
        public static void Notify(this PropertyChangedEventHandler eventHandler, Expression<Func<object>> expression) {
            if (null == eventHandler) {
                return;
            }
            var lambda = expression as LambdaExpression;
            MemberExpression memberExpression;
            if (lambda.Body is UnaryExpression) {
                var unaryExpression = lambda.Body as UnaryExpression;
                memberExpression = unaryExpression.Operand as MemberExpression;
            } else {
                memberExpression = lambda.Body as MemberExpression;
            }
            var constantExpression = memberExpression.Expression as ConstantExpression;
            var propertyInfo = memberExpression.Member as PropertyInfo;

            foreach (PropertyChangedEventHandler del in eventHandler.GetInvocationList()) {
                del(constantExpression.Value, new PropertyChangedEventArgs(propertyInfo.Name));
            }
        }
    }
}