Metro App OnNavigatedTo setting textbox value in a different method - windows

Question

I am following around on a kindle book I bought for developing metro apps. For some reason I cannot set the text value of a text box in a method outside the OnNavigatedTo method. This is the code that the book provides:
protected override void OnNavigatedTo(NavigationEventArgs e)
{
//passed in the view model
viewModel = e.Parameter as ViewModel;
this.DataContext = viewModel;
viewModel.PropertyChanged += (sender, eventArgs) =>
{
if (eventArgs.PropertyName == "SelectedItemIndex")
{
if (viewModel.SelectedIndex == -1)
{
SetItemDetail(null);
}
else
{
SetItemDetail(viewModel.GroceryList[viewModel.SelectedIndex]);
}
}
SetItemDetail(viewModel.GroceryList[viewModel.SelectedIndex]);
};
}
private void SetItemDetail(GroceryItem item)
{
ItemDetailName.Text = "test"; //(item == null) ? "" : item.Name;
ItemDetailQuantity.Text = "test"; //(item == null) ? "" : item.Quantity.ToString();
//if (item != null)
//{
// ItemDetailStore.SelectedItem = item.Store;
//}
//else
//{
// ItemDetailStore.SelectedIndex = -1;
//}
}
I have commented parts out in the set item detail method, but I still cannot set the value of a textbox when I click it (this is supposed to be the behavior). I have used break points and the property of the textbox is getting set, however, it is not displayed on screen.
Thanks.