Don't understand how to use JSON.NET with ASP.NET Core WebAPI

Question

I've completed my first ASP.NET Core Web API and I'd like to try my hand at manually serializing/deserializing JSON via the JSON.NET library. In the JSON.NET documentation they give the following simple manual serialization example:

public static string ToJson(this Person p)
{
    StringWriter sw = new StringWriter();
    JsonTextWriter writer = new JsonTextWriter(sw);

    writer.WriteStartObject();

    // "name" : "Jerry"
    writer.WritePropertyName("name");
    writer.WriteValue(p.Name);

    // "likes": ["Comedy", "Superman"]
    writer.WritePropertyName("likes");
    writer.WriteStartArray();
    foreach (string like in p.Likes)
    {
        writer.WriteValue(like);
    }
    writer.WriteEndArray();

    writer.WriteEndObject();

    return sw.ToString();

}

What's lacking for a beginner such as myself is how to use this string. For example, consider the following:

[HttpGet("/api/data")
[Produces("application/json")]
public IActionResult GetData()
{
    return Ok(new Byte[SomeBigInt]);
}

In the above code I don't really know where ASP.NET Core serializes the array to JSON...I'm assuming it happens somewhere under the hood. If I were to manually serialize (using the JSON.NET example) some big Byte array, what do I do with the resultant string? Is it just "return Ok(myJsonString);"? Won't the built-in serializer - not knowing that it is already the result of a serialization operation- serialize it again?

you can create your own IActionResult derived result where you return your serialized content — Nkosi
– Nkosi, Commented Apr 6, 2017 at 1:20

Roman Pokrovskij · Accepted Answer · 2017-04-23 14:09:54Z

1

Since Asp.Net Core is quite flexible, there are several ways to return JSON. If you want to return Json from a controller one of the most straight forward ways to do it is like this:

 [HttpGet("/api/data")]
 public JsonResult GetData() {

     return Json(new {
        fieldOneString = "some value",
        fieldTwoInt= 2
     });

 }

Under the hood the Json() helper method on the Controlleris using JSON.NET to do the JSON serialization and then sending that as the response body.

You could do the same thing like this:

 string jsonText = JsonConvert.SerializeObject(new {
        fieldOneString = "some value",
        fieldTwoInt= 2
     });

 Response.WriteAsync(jsonText);

Note: to use Response.WriteAsync(jsonText) you need to add using Microsoft.AspNetCore.Http to your file and have a project reference to Microsoft.AspNetCore.Http.Abstractions.

edited Apr 23, 2017 at 14:09

Roman Pokrovskij

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answered Apr 6, 2017 at 12:34

RonC

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10 Comments

LKeene Over a year ago

Thank you for that. I suppose what I'm so confused about is: how do I know apriori what I can and cannot return from my controller. If I return a JsonResult object how do I know that the client will understand what that is? And why haven't you set the return type as "IActionResult" and then "return Ok(anObjectOfTypeJsonResult);"? I'm very confused by the whole thing (as I'm sure is obvious).

RonC Over a year ago

A JsonResult implements IActionResult and the Jaon controller helper method returns a JsonResult. You could change the action method I posted to return an IActionResult and it'd work just as good. I prefer having the action method return JsonResult in this case though since that's the kind of IActionResult the method is going to return. Does that help?

LKeene Over a year ago

Sorry Ron, I'm still pretty confused. Let's say I wished to transmit a raw array of bytes. No serialization to JSON, BSON or whatever...just a Byte array. Could I do this?:

RonC Over a year ago

Absolutely. You could use Response.WriteAsync to send raw bytes if you like via one of its other method signatures.

LKeene Over a year ago

Apologies, having trouble with the posting editor: [HttpGet("/api/data")] public Byte[] GetData() { return Ok(new Byte[10000]); } And then at the client side do: HttpResponseMessage response = httpClient.GetAsyn("/api/data").Result; if(response.IsSuccessStatusCode) { Byte[] someData = response.Content.ReadAsAsync<Byte[]>().Result; } Is there any serialization/deserialization involved in the above, or am I just sending the array of bytes[] out?

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