Error Message While Trying To Run In Windows Phone 7.1 Example Code

The following code is from an example case of how to manage application state in Windows Phone version 7.1 apps.

        // Code to execute when the application is activated (brought to the foreground)
        // This code will not execute when the application is first launched.
        private void Application_Activated(object sender, ActivatedEventArgs e)
        {
          if (e.IsApplicationInstancePreserved)
          {
            ApplicationDataStatus = "application instance preserved.";
            return;
          }
          // Check to see if the key for the application state data is in the State dictionary.
          if (PhoneApplicationService.Current.State.ContainsKey("ApplicationDataObject"))
          {
            // If it exists, assign the data to the application member variable.
            ApplicationDataStatus = "data from preserved state.";
            ApplicationDataObject = PhoneApplicationService.Current.State["ApplicationDataObject"] as string;
          }
        }

When I insert the code above into my example app - per the instructions given in section 6 on the web page - e.IsApplicationInstancePreserved becomes underlined in red. Also when I try to Build the app, I get the following error message in the Error List pane, at the bottom VS 2010 Express for Windows Phone. (Note: I'm using VS 2010 Express for Windows Phone, version  WPDTRTMRel - 30319.174.)

'Microsoft.Phone.Shell.ActivatedEventArgs' does not contain a definition for 'IsApplicationInstancePreserved' and no extension method 'IsApplicationInstancePreserved' accepting a first argument of type 'Microsoft.Phone.Shell.ActivatedEventArgs' could be found (are you missing a using directive or an assembly reference?)

Note: my project has a reference to the component Microsoft.Phone, and I do have the directive 'using Microsoft.Phone.Shell;' at the top of the .cs file, I'm inserting the code from the example app into.