For Packt, AddFriend() in FriendViewModel

FriendViewModel.cs

public async Task AddFriend()
{
  if (settings.User == null)
    throw new Exception("Not logged in.");
  if (string.IsNullOrEmpty(UserName))
    throw new Exception("Username is blank.");
  IsBusy = true; 
  try 
  { 
    var friend = await service
      .AddFriend(settings.User.Name, UserName); 
    //Update our local list of friends 
    var friends = new List<User>(); 
    if (Friends != null)
      friends.AddRange(Friends); 
    friends.Add(friend); 
    Friends =  friends.OrderBy(f => f.Name).ToArray(); 
  } 
  finally 
  { 
    IsBusy =  false; 
  }
}