wpf datagrid get selected items

private void Row_DoubleClick(object sender, MouseButtonEventArgs e)
{
    ClassName classObj = dataGridName.SelectedItem as ClassName;
    string id = classObj.ID;
}

private void DataGrid_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        try
        {
            var row_list = (Item)dg.SelectedItem;
            listblock1.Content = "You Selected: " + row_list.FirstName + " " + row_list.LastName;
        }
        catch { }

    }
    public class Item
    {
        public string FirstName { get; set; }
        public string LastName { get; set; }
    }