Calling Windows.UI.Popups.MessageDialog.ShowAsync throws exception

I'm trying to use Windows 8 desktop application message dialog,
running the following code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

using Windows.UI.Popups;

namespace WpfApplication1
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }
        private async void Button_Click(object sender, RoutedEventArgs e)
        {
            MessageDialog dialog = new MessageDialog("Hope it will work", "TRY");
            await dialog.ShowAsync();
        }
    }
}

But I get the following exception:

"Element not found. (Exception from HRESULT: 0x80070490)"

Can anyone explain why? and how can I solve this issue?

Note: I added the following properties to csproj file:

<TargetPlatformVersion>8.0</TargetPlatformVersion>

And a reference to "Windows.winmd" to the project references

If you take a look here

http://msdn.microsoft.com/library/windows/apps/BR208674

at the bottom you will see :

Minimum supported client Windows 8 [Windows Store apps only]

If you are not developing a Windows Store App (Windows 8 specific app), but a WPF application, use the MessageBox (http://msdn.microsoft.com/en-us/library/ms602949.aspx)

I see you have ran into a confusion. 'Windows 8' there is used as 'Windows Store Apps' ('Windows 8 Apps', 'WinRT Apps').