MVC: How to Return a String as JSON

Question

In an effort to make a progress reporting process a little more reliable and decouple it from the request/response, I am performing the processing in a Windows Service and persisting the intended response to a file. When the client starts polling for updates, the intention is that the controller returns the contents of the file, whatever they are, as a JSON string.

The contents of the file are pre-serialized to JSON. This is to ensure that there is nothing standing in the way of the response. No processing needs to happen (short of reading the file contents into a string and returning it) to get the response.

I initially though this would be fairly simple, but it is not turning out to be the case.

Currently my controller method looks thusly:

Controller

Updated

[HttpPost]
public JsonResult UpdateBatchSearchMembers()
{
    string path = Properties.Settings.Default.ResponsePath;
    string returntext;
    if (!System.IO.File.Exists(path))
        returntext = Properties.Settings.Default.EmptyBatchSearchUpdate;
    else
        returntext = System.IO.File.ReadAllText(path);

    return this.Json(returntext);
}

And Fiddler is returning this as the raw response

HTTP/1.1 200 OK
Server: ASP.NET Development Server/10.0.0.0
Date: Mon, 19 Mar 2012 20:30:05 GMT
X-AspNet-Version: 4.0.30319
X-AspNetMvc-Version: 3.0
Cache-Control: private
Content-Type: application/json; charset=utf-8
Content-Length: 81
Connection: Close

"{\"StopPolling\":false,\"BatchSearchProgressReports\":[],\"MemberStatuses\":[]}"

AJAX

Updated

The following will likely be changed later, but for now this was working when I was generating the response class and returning it as JSON like a normal person.

this.CheckForUpdate = function () {
var parent = this;

if (this.BatchSearchId != null && WorkflowState.SelectedSearchList != "") {
    showAjaxLoader = false;
    if (progressPending != true) {
        progressPending = true;
        $.ajax({
            url: WorkflowState.UpdateBatchLink + "?SearchListID=" + WorkflowState.SelectedSearchList,
            type: 'POST',
            contentType: 'application/json; charset=utf-8',
            cache: false,
            success: function (data) {
                for (var i = 0; i < data.MemberStatuses.length; i++) {
                    var response = data.MemberStatuses[i];
                    parent.UpdateCellStatus(response);
                }
                if (data.StopPolling = true) {
                    parent.StopPullingForUpdates();
                }
                showAjaxLoader = true;
            }
        });
        progressPending = false;
    }
}

Can you not just return the string, you would need to change the return type to string. — RubbleFord, Mar 19 '12 at 20:53
What are you using to make the Ajax call (jQuery, custom, dojo)? Could you provide that code? — Paul, Mar 19 '12 at 20:54
I don't see you using this `path` variable anywhere in your controller action. Where are you reading the contents of the file? All you do is return a JSON with the contents of the `Properties.Settings.Default.EmptyBatchSearchUpdate` property. Also were you aware that you cannot read a file while another thread is writing to it? At least this cannot happen in a safe way. You might very quickly run into race conditions. So I think that your design is flawed from the beginning. — Darin Dimitrov, Mar 19 '12 at 20:56
@Paul I am using Ajax, I will post it in a minute. The code above is edited, I will change it in a sec. The return value seen in the fiddler return accurately represents properly escaped C# string of the JSON object I am attempting to respond with. — CodeWarrior, Mar 19 '12 at 21:03
When the service opens the file for writing, it can optionally allow or disallow concurrent reading with the FileShare options. Similarly, when the web application opens the file for reading, it can allow or disallow concurrent writing. Disallowing concurrent reading/writing will cause an exception on the other thread/process when one thread/process already has the file open, so that will have to be something that the OP will need to address if continuing to pursue this approach. — Dr. Wily's Apprentice, Mar 19 '12 at 21:10
Yeah I have already encountered problems with that, and have taken steps to prevent it in the Windows Service. Thanks! — CodeWarrior, Mar 19 '12 at 21:19

score 153 · Accepted Answer · edited Apr 30 '14 at 08:40

153

The issue, I believe, is that the Json action result is intended to take an object (your model) and create an HTTP response with content as the JSON-formatted data from your model object.

What you are passing to the controller's Json method, though, is a JSON-formatted string object, so it is "serializing" the string object to JSON, which is why the content of the HTTP response is surrounded by double-quotes (I'm assuming that is the problem).

I think you can look into using the Content action result as an alternative to the Json action result, since you essentially already have the raw content for the HTTP response available.

return this.Content(returntext, "application/json");
// not sure off-hand if you should also specify "charset=utf-8" here, 
//  or if that is done automatically

Another alternative would be to deserialize the JSON result from the service into an object and then pass that object to the controller's Json method, but the disadvantage there is that you would be de-serializing and then re-serializing the data, which may be unnecessary for your purposes.

edited Apr 30 '14 at 08:40

Liam

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answered Mar 19 '12 at 21:01

Dr. Wily's Apprentice

10,212
1
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27

4

+1: You also need to set the result's ContentType property to "application/json", since that's something the JsonResult does automatically. – StriplingWarrior Mar 19 '12 at 21:09
@StriplingWarrior good point, I will update my code example to use the other Content method overload. – Dr. Wily's Apprentice Mar 19 '12 at 21:13
1

It is worth noting that `Content` is returning `ContentResult` where `Json` is returning `JsonResult`. – Shoter Apr 24 '18 at 10:36
There's also a signature with encoding: `return this.Content(returntext, "application/json", System.Text.Encoding.UTF8);` – schieferstapel Mar 04 '22 at 10:45
if no encoding is set, the encoding type won't be written. – Ariwibawa Jul 04 '23 at 07:34

Dmitriy Startsev · Answer 2 · 2023-02-15T14:42:44.797

47

You just need to return standard ContentResult and set ContentType to "application/json". You can create custom ActionResult for it:

public class JsonStringResult : ContentResult
{
    public JsonStringResult(string json)
    {
        Content = json;
        ContentType = "application/json";
    }
}

And then return it's instance:

[HttpPost]
public ActionResult UpdateBatchSearchMembers()
{
    string returntext;
    if (!System.IO.File.Exists(path))
        returntext = Properties.Settings.Default.EmptyBatchSearchUpdate;
    else
        returntext = Properties.Settings.Default.ResponsePath;

    return new JsonStringResult(returntext);
}

edited Feb 15 '23 at 14:42

answered Mar 19 '12 at 21:07

Dmitriy Startsev

1,442
10
19

It is, although its biggest benefit is re-use. I only needed this for a single response. If however in the future, I needed it in other places, I would totally go the route of this answer. – CodeWarrior Apr 03 '15 at 02:20
The return type should be ActionResult. With JsonResult does not compile. With ActionResult works. – Sergei Meleshchuk Feb 13 '23 at 20:58

score 6 · Answer 3 · answered Jun 12 '17 at 16:59

Yeah that's it without no further issues, to avoid raw string json this is it.

    public ActionResult GetJson()
    {
        var json = System.IO.File.ReadAllText(
            Server.MapPath(@"~/App_Data/content.json"));

        return new ContentResult
        {
            Content = json,
            ContentType = "application/json",
            ContentEncoding = Encoding.UTF8
        };
    }

NOTE: please note that method return type of JsonResult is not working for me, since JsonResult and ContentResult both inherit ActionResult but there is no relationship between them.

score 2 · Answer 4 · edited Apr 03 '19 at 22:02

2

Use the following code in your controller:

return Json(new { success = string }, JsonRequestBehavior.AllowGet);

and in JavaScript:

success: function (data) {
    var response = data.success;
    ....
}

edited Apr 03 '19 at 22:02

Cody Gray - on strike

239,200
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answered Apr 03 '19 at 21:40

user3652935

21
1

1

Thank you for the new answer, but that doesn't really fit the original question. I had pre-generated JSON data that I wanted to return, not something I wanted represented as a string value of a JSON object. Also, note that in the question I am in an action that is marked explicitly as an HTTP Post. AllowGet will do nothing for that. – CodeWarrior Apr 04 '19 at 20:27

score 0 · Answer 5 · answered Apr 24 '18 at 10:53

All answers here provide good and working code. But someone would be dissatisfied that they all use ContentType as return type and not JsonResult.

Unfortunately JsonResult is using JavaScriptSerializer without option to disable it. The best way to get around this is to inherit JsonResult.

I copied most of the code from original JsonResult and created JsonStringResult class that returns passed string as application/json. Code for this class is below

public class JsonStringResult : JsonResult
    {
        public JsonStringResult(string data)
        {
            JsonRequestBehavior = JsonRequestBehavior.DenyGet;
            Data = data;
        }

        public override void ExecuteResult(ControllerContext context)
        {
            if (context == null)
            {
                throw new ArgumentNullException("context");
            }
            if (JsonRequestBehavior == JsonRequestBehavior.DenyGet &&
                String.Equals(context.HttpContext.Request.HttpMethod, "GET", StringComparison.OrdinalIgnoreCase))
            {
                throw new InvalidOperationException("Get request is not allowed!");
            }

            HttpResponseBase response = context.HttpContext.Response;

            if (!String.IsNullOrEmpty(ContentType))
            {
                response.ContentType = ContentType;
            }
            else
            {
                response.ContentType = "application/json";
            }
            if (ContentEncoding != null)
            {
                response.ContentEncoding = ContentEncoding;
            }
            if (Data != null)
            {
                response.Write(Data);
            }
        }
    }

Example usage:

var json = JsonConvert.SerializeObject(data);
return new JsonStringResult(json);

MVC: How to Return a String as JSON

Controller

Updated

AJAX

Updated

5 Answers5

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