I'd create in code-beind or ViewModel (depending on what are you using), public variable
Public Int32 itemSelectedIndex {get;set;} //This is a public variable, therefore add it inside your class
and bind it to SelectedIndex
as following:
<phone:LongListSelector x:Name="ListeNotes" Height="535" Width="426" HorizontalAlignment="Left" VerticalAlignment="Top" FontSize="36" Margin="54,0,0,0" SelectedIndex ="{Binding itemSelectedIndex, UpdateSourceTrigger = PropertyChanged}">
don't forget to add datacontext reference in code behind
Public void MainWindow()
{
InitializeComponents();
this.DataContext = this;//this makes sure that you can bind public varibles to XAML
}
Afterwards you simply reference itemSelectedIndex anywhere in your code and it will return selected value (e.g. System.Windows.MessageBox.Show(itemSelectedIndex.ToString());