KenKen Solver in WPF

//Terry Spitz 2008

using System;
using System.Collections.ObjectModel;
using System.Linq;
using System.Windows;
using System.Windows.Controls;

namespace KenKen
{
    /// <summary>
    /// Interaction logic for Window1.xaml
    /// </summary>
    public partial class Window1 : Window
    {
        KenKenSolver solver = new KenKenSolver();
        TextBox[,] textboxes = new TextBox[6,6];
        ObservableCollection<Rule> rules;

        public Window1()
        {
            InitializeComponent();
            solver.board = new Board();
            //solver.board.FillTestData();
            rules = new ObservableCollection<Rule>() { new Rule(' ', 0, Rule.Op.plus) };
            RulesGrid.ItemsSource = rules;
            for (int row = 0; row < 6; row++)
                for (int col = 0; col < 6; col++)
                {
                    TextBox t = new TextBox();
                    t.FontSize = 24;
                    t.TextAlignment = TextAlignment.Center;
                    t.MaxLength = 1;
                    Grid.SetColumn(t, col);
                    Grid.SetRow(t, row);
                    BoardGrid.Children.Add(t);
                    textboxes[row, col] = t;
                }
        }

        private void calculate_Click(object sender, RoutedEventArgs e)
        {
            for (int row= 0; row < 6; row++)
                for (int col = 0; col < 6; col++)
                {
                    string val = textboxes[row, col].Text.ToUpper();
                    if (val.CompareTo("A")<0 || val.CompareTo("Z")>0)
                    {
                        MessageBox.Show("Grid must contain letters between A and Z");
                        return;
                    }
                    solver.board.grid[row, col] = val[0];
                }
            solver.board.rules = rules.ToArray();

            try
            {
                solver.Solve();
            }
            catch(Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
            if (solver.solution == null)
                output.Content = "No solution";
            else
                output.Content = solver.solution.Aggregate("", (s, r) => s+r.Aggregate("", (ss,c)=> ss+","+c.ToString())+"\n");
        }

        private void Test_Click(object sender, RoutedEventArgs e)
        {
            solver.board.FillTestData();
            rules = new ObservableCollection<Rule>(solver.board.rules);
            RulesGrid.ItemsSource = rules;
            for (int row = 0; row < 6; row++)
                for (int col = 0; col < 6; col++)
                {
                    textboxes[row, col].Text = solver.board.grid[row, col].ToString();
                }
       }

        private void Clear_Click(object sender, RoutedEventArgs e)
        {
            for (int row = 0; row < 6; row++)
                for (int col = 0; col < 6; col++)
                {
                    textboxes[row, col].Text = "";
                }
            rules.Clear();
        }

    }
}