List<> behave different after looking at it in Debug mode

Question

I access data in a database through a Web-Api and send it back as Json string to an UWP-App.

MessageRepository.cs:

public List<Message> GetByRecipientId(int id)
{
   var listOfMsg = new List<Message>();
   using (var ctx = new ShowcaseContext())
   {
      var recipientWithId = ctx.Recipients.Where(rec => rec.UserId == id);

      foreach (var recipient in recipientWithId.ToList())
      {
         var msgWithId = ctx.Messages.Where(msg => msg.Id == recipient.MessageId);
         listOfMsg.Add(msgWithId.Single());
      }

      return listOfMsg;
   }
}

MessageController.cs:

[Route("api/message/recipient/{id}")]
public IEnumerable<Message> GetByRecipientId(int id)
{
   return messageRepository.GetByRecipientId(id);
}

The problem is that I'm getting an

"Newtonsoft.Json.JsonSerializationException" Error

when trying to return listOfMsg to the UWP-App.
But if I look at the List<> and it items in debug mode, or access a specific message in the List<> before returning it, the error disappeares and I get the right data in the UWP-App.
It seems like the List<> has a different format before accessing it specifically which can't be serialized.
Has anyone an idea why?

Edit
Message.cs:

public class Message
{
    public int Id { get; set; }
    public virtual Collection<Recipient> Recipients { get; set; }
    public MessageTrigger Trigger { get; set; }
    public string Body { get; set; }
    public DateTime CreatedTs { get; set; }
}

The StackTrace is null, but the error message is

The "ObjectContent`1" type failed to serialize the response body for content type "application/json; charset=utf-8".

Debugger "calls" your code when displaying values (e.g. it automatically calls properties or ToString() method). it these calls change the internal status of the object, it can lead to different results. e.g if using lazy loading as @dotctor asked — Gian Paolo, Nov 23 '15 at 10:55
Check if your class Message had looped reference , also try setting dbContext.Configuration.ProxyCreationEnabled = false; — Anshul Nigam, Nov 23 '15 at 10:58
I guess Entity Framework uses lazy loading as default. It's the first time for me using it, but the behavior of the code is like it's using lazy loading. — croxy, Nov 23 '15 at 11:01
Okay after deactivating lazy loading it works all fine, thank you guys. — croxy, Nov 23 '15 at 11:07
You shouldn't disable lazy loading in your application, you should use include for Recipients and Trigger when fetching Messages. — Aleksa, Nov 23 '15 at 11:31

score 2 · Accepted Answer · edited Jun 20 '20 at 09:12

Most probably you have lazy loading enabled. That means that when you access a navigation property, EF will try to hit the database to get the related entity, and this will work if the DbContext is still "alive".

After the API call returns the data, JSON.NET will visit all the properties to serialize them, including the navigation properties. And, at that point, the DbContext has already been disposed of. That's why you get your error.

To avoid it you can:

disable lay loading for a particular query: Entity Framework: How to disable lazy loading for specific query?
completely disable it, if you're not using it: Disable lazy loading by default in Entity Framework 4

For more info on disabling lazy loading.

Alternative: Json configuration

You can also instruct JSON.NET to skip some properties. But there are mabny ways of doing so, and you have to add attributes to your entities. For example:

Alternative: projection to anonymus class with `.Select()`

And I forgot a final, easy option: you can project your query to an anonymous class, getting read of all problems, and returning the JSON that you exactly want to return.

.Select(new { /* your projection */ });

Although this answers are for older versions, it still works in the same way.

NOTE: while you're debugging you're still inside the DbContext using block, and that's why you don't get errors while debugging. If you put this line: return listOfMsg; outside the using block, add a breakpoint in that line, and watch navigation properties, you'll also get errors in the debugger.

List<> behave different after looking at it in Debug mode

1 Answers1

Alternative: Json configuration

Alternative: projection to anonymus class with .Select()

Alternative: projection to anonymus class with `.Select()`