MVC 4 Beta - You must write an attribute 'type'='object' after writing the attribute with local name '__type'

Question

I am working with the WebAPI in the new MVC 4 Beta. I came across this error when trying to get an entity that has a virtual ICollection<> property to populate. Is there a way to get around this as of right now? I understand this is in the Beta stage, so this might be fixed down the road. It was just a curiosity if there is a solution out there for this.

score 2 · Accepted Answer · answered Mar 13 '12 at 11:40

I got this to work by removing the virtual keyword and making sure that the objects and collections that did have a virtual keyword were provided in the Include statement within my repository.

public class Order
{
    public int ID { get; set; }
    public DateTime OrderDate { get; set; }
    public ICollection<Product> Products { get; set; }
}

public interface IOrderRepository
{
    IQueryable<Order> Orders { get; }
    void SaveOrder(Order order);
    void DeleteOrder(Order order);
}

public class OrderRepository
{
    StoreDbContext db = new StoreDbContext();

    public IQueryable<Order> Orders
    {
        get { return db.Orders.Include("Products"); }
    }

    public void SaveOrder(Order order)
    {
        db.Entry(order).State = order.ID == 0 ? 
                EntityState.Added : 
                EntityState.Modified;

        db.SaveChanges();
    }

    public void DeleteOrder(Order order)
    {
        db.Orders.Remove(order);

        db.SaveChanges();
    }
}

If the error is related to serializing the auto-generated proxies AND if you don't really need them then you might just need to disable proxy creation on your context... Here is a similar Q/A...http://stackoverflow.com/questions/8173524/webapi-with-ef-code-first-generates-error-when-having-parent-child-relation — blins, Apr 16 '12 at 18:45
Here's another solution which probably is more efficient http://stackoverflow.com/questions/7235650/entity-framework-serialize-poco-to-json — starcorn, Jul 20 '12 at 08:25

score 1 · Answer 2 · answered Jun 15 '12 at 10:33

I had the same problem, it seems to be a problem with the default WebApi serializer. I added Json.Net as a formatter in my Global.asax.cs and it worked fine for me. I just followed this example: http://blogs.msdn.com/b/henrikn/archive/2012/02/18/using-json-net-with-asp-net-web-api.aspx

This is what I've got in my Global.asax.cs

JsonSerializerSettings serializerSettings = new JsonSerializerSettings();
serializerSettings.Converters.Add(new IsoDateTimeConverter());
GlobalConfiguration.Configuration.Formatters[0] = new JsonNetFormatter(serializerSettings);

I just added the Json.Net package using NuGet and created the JsonNetFormatter class as explained in the post above.

score 0 · Answer 3 · answered May 11 '12 at 18:16

I had a similar problem. I fixed it by using a sort of ViewModel class that only had simple types. I translated the object returned by the DbContext into my ViewModel class and passed that back to the client.

This wouldn't work in all situations, but it did in mine.

score 0 · Answer 4 · answered Jul 06 '12 at 14:38

Ran into this problem also. My situation was a little different though.

I had this structure and it would not work for me.

[DataContract]
public class MJPEGCamera : Camera
{
}
[DataContract]
public class H264Camera : Camera
{     
}
[DataContract]
public class Camera
{
    [DataMember]
    public string cameraName { get; set; }
    [DataMember]
    public string address { get; set; }
    [DataMember]
    public string format { get; set; }
    [DataMember]
    public string archiveDaysUrl { get; private set; }
    [DataMember]
    public string archiveHoursUrl { get; private set; }
}

So I just created a factory in camera to accomplish what I needed. Hope this will help somebody who finds this answer.

MVC 4 Beta - You must write an attribute 'type'='object' after writing the attribute with local name '__type'

4 Answers4