Migrate from Basic to MVVM and MEF Composable Patterns for a Silverlight Application - Part 1

using System;
using System.Collections.Generic;
using System.Linq;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Media;

namespace ProductApp.Views
{
    public partial class AddProductWindow : ChildWindow
    {
        public AddProductWindow(IEnumerable<string> categories)
        {
            InitializeComponent();
            Categories = categories;
            categoryList.ItemsSource = this.Categories;
            categoryList.IsEnabled = false;
            OKButton.IsEnabled = false;
        }

        public string ProductName { get; set; }
        public decimal Price { get; set; }        
        public IEnumerable<string> Categories { get; set; }
        public string SelectedCategory { get; set; }

        public event EventHandler AddEvent;

        private void OKButton_Click(object sender, RoutedEventArgs e)
        {
            ProductName = productName.Text;

            if (System.Text.RegularExpressions.Regex.IsMatch(unitPrice.Text, @"^-?\d*[0-9]?(|.\d*[0-9]|,\d*[0-9])?$"))
            {
                Price = Convert.ToDecimal(unitPrice.Text);
            }
            else
            {
                MessageBox.Show("Invalid Price");
                unitPrice.Background = new SolidColorBrush(Colors.Red);
                return;
            }
            SelectedCategory = this.categoryList.SelectedItem as string;
            DialogResult = true;
            AddEvent(this, new EventArgs());
        }

        private void CancelButton_Click(object sender, RoutedEventArgs e)
        {
            this.DialogResult = false;
        }

        private void productName_TextChanged(object sender, TextChangedEventArgs e)
        {
            CheckForOKButton();
        }

        private void unitPrice_TextChanged(object sender, TextChangedEventArgs e)
        {
            CheckForOKButton();
        }
        
        private void unitPrice_GotFocus(object sender, RoutedEventArgs e)
        {
            unitPrice.Background = new SolidColorBrush(Colors.White);            
        }

        private void categoryList_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            CheckForOKButton();
        }

        private void CheckForOKButton()
        {
            if (productName.Text == "" || unitPrice.Text == "" || categoryList.SelectedIndex < 1)
            {
                OKButton.IsEnabled = false;
            }
            else
            {
                OKButton.IsEnabled = true;
            }
        }        
    }
}