Application NavigationFailed event

   
  
<Application x:Class="NavigationApplication.App"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    StartupUri="StartupWindow.xaml" NavigationFailed="App_NavigationFailed">
</Application>

//File:Window.xaml.cs

using System;
using System.Windows;
using System.Data;
using System.Xml;
using System.Configuration;
using System.Windows.Threading;
using System.Windows.Navigation;

namespace NavigationApplication
{
    public partial class App : System.Windows.Application
    {
        private void App_NavigationFailed(object sender, NavigationFailedEventArgs e)
        {
            if (e.Exception is System.Net.WebException)
            {                
                MessageBox.Show("Website " + e.Uri.ToString() + " cannot be reached.");
                e.Handled = true;                
            }
        }     
    }
}