实体框架.find（id）NullReferenceException

Question

I am working through the MVC Music Store tutorial , and have come across a small problem / query.

I am looking to find an album by it's id.

var album = db.Albums.Find(id)

This works fine however, if the id does not exist, then I get a NullReferenceException when the controller passes a null model to the view.

I can think of 2 ways to fix this.

Method 1: Check for null in controller, if null, show a different view / redirect to a different action such as an index page or dedicated error page / 404 not found page.

    //
    // GET: /Store/Details/4
    public ActionResult Details(int? id)
    {
        var album = db.Albums.Find(id);

        if (album == null) return RedirectToAction("Index");
        else return View(album);
    }

Method 2: Check if model is null in the view, if so, don't show model specific items, but instead show an error message.

@model MvcMusicStore.Models.Album
@{
    ViewBag.Title = "Details";
}
@if (Model == null)
{
    <h2>That album doesn't exist</h2>
} 
else 
{
    <h2>Details: @Model.Title</h2>
}

The Question: Are there any other ways / best practice ways of dealing with this? Are there any advantages of method 1, vs method 2?

Answer 1

You could write a custom action filter that will check if the model passed to the view is null and render a 404 page. This way you don't need to repeat this logic in all controller actions:

public class CheckForEmptyModelAttribute: ActionFilterAttribute
{
    public override void OnResultExecuted(ResultExecutedContext filterContext)
    {
        var viewResult = filterContext.Result as ViewResultBase;
        if (viewResult != null && viewResult.Model == null)
        {
            var view404 = new ViewResult
            {
                ViewName = "~/Views/Shared/404.cshtml"
            };
            filterContext.Result = view404;
        }
    }
}

and then:

//
// GET: /Store/Details/4
[CheckForEmptyModel]
public ActionResult Details(int? id)
{
    var album = db.Albums.Find(id);
    return View(album);
}

Alternative approach consists in writing a custom route by deriving from the Route class in which you would retrieve the model and if it is not found simply do not match the route.

Answer 2

//
// GET: /Store/Details/4
public ActionResult Details(int? id)
{
    var album = db.Albums.Find(id);
    return ViewIfNotNull(album);
}

// boxing
private ActionResult ViewIfNotNull(object model)
{
    if (album == null) return RedirectToAction("Index");
    else return View(album);
}

// OR generic
private ActionResult ViewIfNotNull<T>(T model)
{
    if (album == null) return RedirectToAction("Index");
    else return View(album);
}