web api json deserialization failure - asp.net-mvc

Question

I have a request endpoint using in web api:
public HttpResponseMessage Create(IList<SlideContent> l)
{
...
}
i send the parameter as a json and web api serializes it to IList
SlideContent is:
public abstract class SlideItem
{
...
}
and i have specialised classes
public class TitleSlideItem : SlideItem
{
...
}
public class ParagraphSlideItem : SlideItem
{
...
}
just like that i can't call the Create function, because i get a
missingmethodexception: cannot create abstract class
so i can't deserialize the json parameter. if i remove the abstract modifier, then i don't have specialized objects, every object's type will be SlideContent.
I even put annotations in the json, but it doesn't help either.
If i'm not wrong, the i would have to write a custom binder for the abstract class, but how can i do that?
Sincerely,
Zoli

Answer 1

One possibility is to substitute the built-in JSON serializer with a custom formatter using JSON.NET as shown in the following blog post.
public class JsonNetFormatter : MediaTypeFormatter
{
private JsonSerializerSettings _jsonSerializerSettings;
public JsonNetFormatter(JsonSerializerSettings jsonSerializerSettings)
{
_jsonSerializerSettings = jsonSerializerSettings ?? new JsonSerializerSettings();
// Fill out the mediatype and encoding we support
SupportedMediaTypes.Add(new MediaTypeHeaderValue("application/json"));
Encoding = new UTF8Encoding(false, true);
}
protected override bool CanReadType(Type type)
{
if (type == typeof(IKeyValueModel))
{
return false;
}
return true;
}
protected override bool CanWriteType(Type type)
{
return true;
}
protected override Task<object> OnReadFromStreamAsync(Type type, Stream stream, HttpContentHeaders contentHeaders, FormatterContext formatterContext)
{
// Create a serializer
JsonSerializer serializer = JsonSerializer.Create(_jsonSerializerSettings);
// Create task reading the content
return Task.Factory.StartNew(() =>
{
using (StreamReader streamReader = new StreamReader(stream, Encoding))
{
using (JsonTextReader jsonTextReader = new JsonTextReader(streamReader))
{
return serializer.Deserialize(jsonTextReader, type);
}
}
});
}
protected override Task OnWriteToStreamAsync(Type type, object value, Stream stream, HttpContentHeaders contentHeaders, FormatterContext formatterContext, TransportContext transportContext)
{
// Create a serializer
JsonSerializer serializer = JsonSerializer.Create(_jsonSerializerSettings);
// Create task writing the serialized content
return Task.Factory.StartNew(() =>
{
using (JsonTextWriter jsonTextWriter = new JsonTextWriter(new StreamWriter(stream, Encoding)) { CloseOutput = false })
{
serializer.Serialize(jsonTextWriter, value);
jsonTextWriter.Flush();
}
});
}
}
then in Application_Start when registering the formatter you could configure the serializer to use type information in the JSON:
var formatters = GlobalConfiguration.Configuration.Formatters;
formatters.Remove(formatters.XmlFormatter);
formatters.Remove(formatters.JsonFormatter);
var serializerSettings = new JsonSerializerSettings();
serializerSettings.TypeNameHandling = TypeNameHandling.Objects;
serializerSettings.Converters.Add(new IsoDateTimeConverter());
formatters.Add(new JsonNetFormatter(serializerSettings));
and then you could POST the following JSON:
[
{
"$type":"AppName.Models.TitleSlideItem, AppName",
"Id":1,
"Title":"some title" // this is a specific property of the TitleSlideItemclass
},
{
"$type":"AppName.Models.ParagraphSlideItem, AppName",
"Id":2,
"Paragraph":"some paragraph" // this is a specific property of the ParagraphSlideItem class
}
]
which will be successfully deserialized inside this action:
public HttpResponseMessage Post(IList<SlideItem> l)
{
...
}

Answer 2

JsonFx creates custom binders. For example take a look at:
https://code.google.com/p/jsonfx/source/browse/trunk/JsonFx/JsonFx.MvcTemplate/Global.asax.cs
Look at the RegisterBinders() function in that file and the source to the binders or just install JsonFx and create a project using their MVC template and see how they do it or just use their library if that would work for you. That's what I would do.