After you setup your development environment create a new Silverlight 4 application using Visual Studio 2010. This will automatically create a page "MainPage.xaml" for you. Add two buttons in your XAML. One is to install the Silverlight application as out of browser & another is to show the notification.
<Button x:Name="btnInstall" Width="150" Height="20" Content="Install OOB" Margin="5"/>
<Button x:Name="btnShowNotification" Width="150" Height="20" Content="Show Notification" Margin="5"/>
For implementing the Silverlight out of browser feature follow my earlier post: "How can you implement Silverlight 3 Out-Of-Browser feature?" Now go to the code behind file "MainPage.xaml.cs" & implement the click event for those. On page load, if it is running out of browser then create the notification window instance:
// Initialize a new instance of Notification Window
notificationWindow = new NotificationWindow();
notificationWindow.Height = 50.0;
notificationWindow.Width = 300.0;
Create your custom notification panel either using XAML or code behind. You can go for a nice looking UserControl. Here for example I will use a TextBlock inside a Border to show a simple message.
Border border = new Border()
Background = new SolidColorBrush(Colors.Gray),
Height = notificationWindow.Height,
Width = notificationWindow.Width,
Child = new TextBlock()
Text = "This is a Custom Notification from Silverlight 4",
Foreground = new SolidColorBrush(Colors.White)
Now, on "Show Notification" button click event check for whether it is running out of browser. If so, assign the notification panel you created to the content of the notification window instance & call the show method of the notification window. Here you have to pass the duration of the visibility of the notification in milliseconds.
notificationWindow.Content = border; // add the custom notification panel
notificationWindow.Show(2000); // show the notification
Now run your application & click on the "Install OOB" button. This will install the Silverlight application and open the desktop version of it. Click the "Show Notification" button to show the notification at the right bottom corner of your desktop.