How to POST in ASP.NET Web API with model property as interface

Question

Suppose I have a model:

public class Menu
{
    public string Name { get; set; }
    public IMenuCommand Next { get; set; }
}

IMenuCommand could have different implementations, like:

public class NextStepCommand : IMenuCommand
{
    public int Step { get; set; }
}

public class VoiceCommand : IMenuCommand
{
    public string Message { get; set; }
}

And I want to POST menus with different commands to the ASP.NET Web API service. How can I do that?

The request below will create an object with specified Name, but Next command will be null:

POST http://localhost/api/menus: {"name":"bob","next":{"step":1}}
Returns 201: {"Name":"bob","Next":null}

Default Web API binders can't map my request params to the needed C# type - of course it's a tricky part. Can I use some "known-type" attribute for interface-based properties or is there any other approach to handle this case, probably a custom model binder?

abatishchev · Accepted Answer · 2014-07-01 17:53:07Z

5

I think what you're looking for is Json.NET's support for type name handling. It allows you to specify the type to deserialize into by adding the "$type" json tag. You can try this code out to see how it works:

Console.WriteLine(JsonConvert.DeserializeObject<Menu>(
    @"{
         ""name"":""bob"",
         ""next"":
         {
           ""$type"" : ""ConsoleApplication.NextStepCommand,ConsoleApplication"",
           ""step"" : 1
         }
    }",
    new JsonSerializerSettings() { TypeNameHandling = TypeNameHandling.Auto }).Next);

You'll have to replace the namespace and assembly name with your own, but you should see the NextStepCommand being correctly deserialized.

In WebAPI, you'll need to tweak your request to add the "$type" type information, and you'll need to enable TypeNameHandling like this:

config.Formatters.JsonFormatter.SerializerSettings.TypeNameHandling = TypeNameHandling.Auto;

edited Jul 1, 2014 at 17:53

abatishchev

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answered Dec 19, 2012 at 3:23

Youssef Moussaoui

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5 Comments

whyleee Over a year ago

Worked for me when I set TypeNameHandling.Auto for Web API JSON serializer and sent a request with needed "$type" of a command. But can I turn on TypeNameHandling feature only for deserialization? Because now I have too much garbage in JSON output in GET services.

whyleee Over a year ago

Found that it's possible to use [JsonProperty(TypeNameHandling = TypeNameHandling.All)] for specific model properties (from Json.NET blog). Checked with Web API - it works. Much better now.

amcdnl Over a year ago

@Youssef Moussaoui - Any ideas on this question: stackoverflow.com/questions/20252027/… ... Not having any luck with anything!

user384080 Over a year ago

@youssef - "you'll need to tweak your request to add the "$type" type information" --> please explain more how to implement this in WEB API ?

newman Over a year ago

I added TypeNameHandling.Auto to my WebApiConfig for my Asp.Net project, but I don't see it working. However, it tested out with JsonConverter with this setting, it does work. What did I miss?

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