WPF/Silverlight: Step By Step Guide to MVVM

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

namespace MVVMDemo.Model
{
    public class Employee : IDataErrorInfo, INotifyPropertyChanged
    {
        #region Constructor
        /// <summary>
        /// Constructor for Employee class with default parameters
        /// </summary>
        /// <param name="id"></param>
        /// <param name="name"></param>
        /// <param name="age"></param>
        /// <param name="notes"></param>
        public Employee(string id = "", string name = "", uint age = 0, string notes = "")
        {
            _id = id;
            _name = name;
            _age = age;
            _notes = notes;
        } 
        #endregion

        #region Properties
        private string _id = string.Empty;

        public string ID
        {
            get { return _id; }
            set
            {
                _id = value;
                RaisePropertyChanged("ID");
            }
        }

        private string _name = string.Empty;

        public string Name
        {
            get { return _name; }
            set
            {
                _name = value;
                RaisePropertyChanged("Name");
            }
        }

        private uint _age = 0;

        public uint Age
        {
            get { return _age; }
            set
            {
                _age = value;
                RaisePropertyChanged("Age");
            }
        }

        private string _notes = string.Empty;

        public string Notes
        {
            get { return _notes; }
            set
            {
                _notes = value;
                RaisePropertyChanged("Notes");
            }
        } 
        #endregion

        #region IDataErrorInfo
        public string Error
        {
            get
            {
                return this[string.Empty];
            }
        }

        public string this[string propertyName]
        {
            get
            {

                string result = string.Empty;
                propertyName = propertyName ?? string.Empty;

                if (propertyName == string.Empty || propertyName == "ID")
                {
                    if (string.IsNullOrEmpty(this.ID))
                    {
                        result = "Employee ID is invalid. ID cannot be null or blank";
                    }
                    else if (System.Text.RegularExpressions.Regex.IsMatch(this.ID, "[/!@#?/}[}{ ]"))
                    {
                        result = "Employee ID is invalid. ID cannot have special characters";
                    }
                }
                else if (propertyName == "Name")
                {
                    if (string.IsNullOrEmpty(this.Name))
                    {
                        result = "Name is invalid. ID cannot be null or blank";
                    }
                }
                else if (propertyName == "Age")
                {
                    if (Age > 150 || Age < 0)
                    {
                        result = "Age is invalid. Age should be between 0 and 150";
                    }
                }

                return result;
            }
        }
        #endregion

        #region INotifyPropertyChanged
        [field: NonSerialized]
        public event PropertyChangedEventHandler PropertyChanged;

        protected void RaisePropertyChanged(string propertyName)
        {
            var handler = PropertyChanged;
            if (handler != null)
            {
                handler(this, new PropertyChangedEventArgs(propertyName));
            }
        }
        #endregion
    }
}