public IActionResult OnGet(int?isn)
{
if (isn.HasValue)
{
this.Book = BookData.GetByISN(isn.Value);
}
else
{
this.Book = new Book();
}
this.Categories = HtmlHelper.GetEnumSelectList <Category> ();
if (Book == null)
{
return(RedirectToPage("./NotFound"));
}
return(Page());
}