Is there a better way to cast an JObject to any implementation of an interface?

Question

I am not sure if there's a better way to do this. maybe someone help?

I want to cast an object of type JObject to a class in a factory. Class itself should be decided based on on another parameter. But I can only think of Serializing the object to a string an serializing back into a specific class. There has to be a better way?

https://dotnetfiddle.net/3Qwq6V

using Newtonsoft.Json;
using Newtonsoft.Json.Serialization;
using System;

namespace Test
{
    public class Input
    {
        public int TypeId { get; set; }
        public object ObjectDefinesInput;
    }

    public class VoiceInput
    {
        public string Language;
    }

    public class TextInput
    {
        public string Encoding;
    }

    public interface IResponse
    {
        void Respond();
    }

    public class VoiceResponse : IResponse
    {
        private VoiceInput input { get; set; }
        public VoiceResponse(VoiceInput input) { this.input = input; }

        public void Respond()
        {
            // use information on VoiceInput to do something
            Console.WriteLine("(In "+ this.input.Language +"): beep buup boop.");
        }
    }

    public class TextResponse : IResponse
    {
        private TextInput input { get; set; }
        public TextResponse(TextInput input) { this.input = input; }
        public void Respond()
        {
            Console.WriteLine("I am a text handler. Using "+ this.input.Encoding +".");
        }
    }

    public static class ResponseFactory
    {
        public static IResponse CreateResponseHandler(Input input)
        {
            // ----------------- ISSUE HERE -----------------------------//
            // I'm using JsonConvert to serialize an <object> to a string, and then 

            string jsonObjectDefinesInput = JsonConvert.SerializeObject(input.ObjectDefinesInput, new JsonSerializerSettings
            {
                Formatting = Formatting.Indented,
                ContractResolver = new CamelCasePropertyNamesContractResolver()
            });

            switch (input.TypeId)
            {
                case 1:
                    // (VoiceInput) input.ObjectDefinesInput throws exception
                    // input.ObjectDefinesInput as VoiceInput returns null
                    VoiceInput voiceInput = JsonConvert.DeserializeObject<VoiceInput>(jsonObjectDefinesInput);
                    return new VoiceResponse(voiceInput);
                default:
                    TextInput textInput = JsonConvert.DeserializeObject<TextInput>(jsonObjectDefinesInput);
                    return new TextResponse(textInput);
            }
        }
    }

    public class Program
    {
        public static void Main(string[] args)
        {
            string jsonData1 = "{ \"typeId\": 1, \"ObjectDefinesInput\": { \"Language\": \"Robot\" } }";
            string jsonData2 = "{ \"typeId\": 2, \"ObjectDefinesInput\": { \"Encoding\": \"UTF-8\" } }";

            Input someInpput1 = JsonConvert.DeserializeObject<Input>(jsonData1);
            Input someInpput2 = JsonConvert.DeserializeObject<Input>(jsonData2);

            IResponse testResponse1 = ResponseFactory.CreateResponseHandler(someInpput1);
            IResponse testResponse2 = ResponseFactory.CreateResponseHandler(someInpput2);

            testResponse1.Respond();

            testResponse2.Respond();

            Console.ReadLine();
        }
    }
}

Pls provide the code inside this question, so if someone looks at your question in 3 years also sees the code belonging to it. External resources may get deleted by times. — Tobias Theel, Sep 01 '17 at 15:37
When you say "object of type object", should we assume you mean "reference of type object to something definitely *not* of type object"? Because if the actual runtime type of the object is `object`, there's little future in casting it to anything else. I glanced at your fiddle but didn't see any indication of where the relevant code is. — 15ee8f99-57ff-4f92-890c-b56153, Sep 01 '17 at 15:38
@EdPlunkett assume I have Deserialized Json into type object. There is no underlying class (other than maybe JObject returned by JsonConvert?) — Evaldas Raisutis, Sep 01 '17 at 15:41
The runtime type of any object `o` can be determined by calling `o.GetType()`. — 15ee8f99-57ff-4f92-890c-b56153, Sep 01 '17 at 15:42
@EdPlunkett It's just {Name = "JToken" FullName = "Newtonsoft.Json.Linq.JToken"} — Evaldas Raisutis, Sep 01 '17 at 15:44
Why are you serializing and deserializing `input` in `CreateResponseHandler`? What is the goal there? What do you get from `input.GetType()`? Are you saying that you just want `case 1: return new VoiceResponse((VoiceInput)input);`? — 15ee8f99-57ff-4f92-890c-b56153, Sep 01 '17 at 15:45
@EdPlunkett I want to return an implementation of the interface based on the TypeId on the input class and pass a parameter into the constructor of that implementation class. Without knowing how the input is structured when I call the factory method. This way any of the implementation classes can rely on receiving information required to run from the object — Evaldas Raisutis, Sep 01 '17 at 15:50
@EdPlunkett (VoiceInput)input will throw null exception, but yes basically that is what I want. — Evaldas Raisutis, Sep 01 '17 at 15:51
Whoops, I missed a bit of your code that's hidden by the horizontal scrolling. What I should have suggested was `case 1: return new VoiceResponse((VoiceInput)input.ObjectDefinesInput);` -- `input.ObjectDefinesInput` is what you're serializing and then deserializing. — 15ee8f99-57ff-4f92-890c-b56153, Sep 01 '17 at 15:53
I'm guessing a bit though since I have only a tenuous inference about what `input.ObjectDefinesInput` is. — 15ee8f99-57ff-4f92-890c-b56153, Sep 01 '17 at 15:54
Does "throws null" mean "throws NullReferenceException"? What is `input.ObjectDefinesInput` before you serialize it? What is its value? Is it null? If not null, what does `input.ObjectDefinesInput.GetType()` return? — 15ee8f99-57ff-4f92-890c-b56153, Sep 01 '17 at 15:54

score 2 · Answer 1 · answered Sep 01 '17 at 15:55

2

If you just supplied the right sort of input, you could cast them:

var voiceInput = new Input()
{
    TypeId = 1,
    ObjectDefinesInput = new VoiceInput(){ ... }
}

and

switch (input.TypeId)
{
    case 1:
          VoiceInput voiceInput = (VoiceInput)input.ObjectDefinesInput;
          return new VoiceResponse(voiceInput);
    default:
          TextInput textInput = (textInput)input.ObjectDefinesInput;
          return new TextResponse(textInput );
}

If you want some type safety, make your Input class have a generic type argument for the type of input

public class Input<T>
{
    public int TypeId { get; set; }
    public T ObjectDefinesInput;
}

And

var voiceInput = new Input<VoiceInput>()
{
    TypeId = 1,
    ObjectDefinesInput = new VoiceInput(){ ... }
}

Then no casting required:

switch (input.TypeId)
{
    case 1:
          return new VoiceResponse(input.ObjectDefinesInput);
    default:
          return new TextResponse(input.ObjectDefinesInput);
}

answered Sep 01 '17 at 15:55

Jamiec

133,658
13
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I don't know which class ObjectDefinesInput should be cast to until I switch on input.TypeId. – Evaldas Raisutis Sep 01 '17 at 15:57
@EvaldasRaisutis indeed. I thought that was the point of the question. – Jamiec Sep 01 '17 at 16:01
1

I can't do new Input, because Input is deserialized from JSON. – Evaldas Raisutis Sep 01 '17 at 16:05
@EvaldasRaisutis your question said you were just serializing and deserializing because you couldnt work out how to cast. I thought the question was how to *not* do that. My mistake. – Jamiec Sep 01 '17 at 16:13

score 1 · Accepted Answer · answered Sep 01 '17 at 16:02

1

Hmm, since I found out the type is actually JObject (as I deserialize into < object >, the underlying class is JObject), then I can do

input.ObjectDefinesInput.ToObject<TextInput>();

as per Converting a JToken (or string) to a given Type

answered Sep 01 '17 at 16:02

Evaldas Raisutis

1,628
4
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1

I finally pasted your code into VS and got it through my head what the real issue is. This is a nice feature I hadn't known about. +1. May I suggest retitling your question to something like "How to cast JObject to actual type that was serialized"? – 15ee8f99-57ff-4f92-890c-b56153 Sep 01 '17 at 16:07

Is there a better way to cast an JObject to any implementation of an interface?

2 Answers2