ASP.NET Web-API not serializing readonly property

Question

I migrated an API method from a standard MVC action to the new asp.net Web-API beta and suddenly a read only property is no longer serialized (both returning JSON). Is this expected behaviour?

Edit: Added code sample

I have both Newtonsoft.Json 4.0.8 and System.Json 4.0 referenced through nuget packages

        public IQueryable<Car> Gets()
        {
             return _carRepository.GetCars();
        }

        public class Car
        {
            public IEnumerable<Photo> Photos
            {
                get { return _photos; }
            }

            public string PreviewImageUrl // No longer serialized
            {
                get
                {
                     var mainImage = Photos.FirstOrDefault(o => o.IsMainPreview) Photos.FirstOrDefault();
                        return mainImage != null ? mainImage.Url : (string.Empty);
                    }
                }
             }
         }

Answer 1

The JsonMediaTypeFormatter that ships with the Beta uses a serializer that does not support read-only properties (since they would not round-trip correctly). We are planning on addressing this for the next realese.

In the mean-time you could use a custom JSON MediaTypeFormatter implementation that uses Json.NET (there's one available here) instead of the built-in formatter.

Update: Also check out Henrik's blog about hooking up a JSON.NET formatter: http://blogs.msdn.com/b/henrikn/archive/2012/02/18/using-json-net-with-asp-net-web-api.aspx

Answer 2

I don't know if this is an expected behavior or not. I would say that this is expected for input parameters (because you cannot set their values) but not for output parameters. So I would say this is a bug for an output parameter. And here's an example illustrating the issue:

Model:

public class Product
{
    public Product()
    {
        Prop1 = "prop1 value";
        Prop2 = "prop2 value";
        Prop3 = "prop3 value";
    }

    public string Prop1 { get; set; }

    [ReadOnly(true)]
    public string Prop2 { get; set; }

    public string Prop3 { get; protected set; }
}

Controller:

public class ProductsController : ApiController
{
    public Product Get(int id)
    {
        return new Product();
    }
}

Request:

api/products/5

Result:

{"Prop1":"prop1 value","Prop2":"prop2 value"}

So if the property doesn't have a public setter it is not serialized which doesn't seem normal as the Product class is used as output in this case.

I would suggest opening a connect ticket so that Microsoft can fix this before the release or at least tell that this is by design.