C#

using System;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Controls.Primitives;
using System.Windows.Media;

namespace CodeOnlyWPF {

   class Program {

      [STAThread]
      static void Main( string[] args ) {

         Window window = new Window() {
            WindowStartupLocation = WindowStartupLocation.CenterScreen,
            SizeToContent = SizeToContent.WidthAndHeight
         };

         UniformGrid ug = new UniformGrid() {
            Columns = 10,
            Rows = 8
         };

         for ( int i = 1; i <= 80; i++ ) {

            Button btn = new Button() {
               Content = i.ToString(),
               Width = 40,
               Height = 40,
               Background = Brushes.GhostWhite
            };

            btn.Click += new RoutedEventHandler( btn_Click );

            ug.Children.Add( btn );

         }  // end for.

         window.Content = ug;

         Application app = new Application();

         app.Run( window );
      }

      static void btn_Click( object sender, RoutedEventArgs e ) {

         Button btn = sender as Button;

         if ( btn.Tag == null ) {
            btn.Background = Brushes.Gold;
            btn.Tag = true;   // Selected.
         }
         else {
            btn.Background = Brushes.GhostWhite;
            btn.Tag = null;   // Not selected.
         }
      }

   }  // end class.

}  // end namespace.

public class MyUpDown : NumericUpDown
    {
    public override void UpButton()
        {
        decimal save = Increment;
        if ((Control.ModifierKeys & Keys.Shift) == Keys.Shift)
            {
            Increment *= 10;
            }
        base.UpButton();
        Increment = save;
        }
    public override void DownButton()
        {
        decimal save = Increment;
        if ((Control.ModifierKeys & Keys.Shift) == Keys.Shift)
            {
            Increment *= 10;
            }
        base.DownButton();
        Increment = save;
        }
    }