Attached Command for Windows 8 Metro Style in C#

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Windows.Foundation;
using Windows.UI.Popups;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Data;
using Windows.UI.Xaml.Input;

namespace Application3
{
    partial class MainPage
    {
        public MainPage()
        {
            InitializeComponent();
            this.DataContext = new VM();
        }
    }

    public class VM
    {
        public DelegateCommand TestCommand
        {
            get
            {
                return new DelegateCommand(p =>
                {
                    MessageDialog messageDialog = new MessageDialog("Testing dialog");
                    var op = messageDialog.ShowAsync();
                    op.Start();
                });
            }
        }
    }
}