.Net Core Model Binding JSON Post To Web API

Question

Just started a new project using .NET Core. Added my Web API controller and related method. Using Postman I created a JSON object and posted it to my controller method. Bear in mind the JSON object matches the Object param in the controller method exactly.

In debug mode I can see the object, it is not null, the properties are there, HOWEVER the prop values are defaulted to their representatives types, 0 for int, etc.

I've never seen this behavior before… so I took exactly the same code and object and replicated in a MVC project with a Web API 2 controller and it works perfectly.

What am I missing, can I not POST JSON and model bind in .NET Core?

Reading this article it seems I cannot unless I send as form POST or as querystring vars which by the way works fine.

https://lbadri.wordpress.com/2014/11/23/web-api-model-binding-in-asp-net-mvc-6-asp-net-5/

JSON:

{
   "id": "4",
   "userId": "3"
   "dateOfTest": "7/13/2017"
}

Method:

[HttpPost]
[Route("test1")]
[AllowAnonymous]
public IActionResult Test(Class1 data)
{
    return Ok();
}

Just wanted to add, if you are using core 3.1 and above, you default to using the System.Text.Json for your conversions. As of now, it only serializes/deserializes properties only and so your class need to use properties instead of fields as in the answer by edincorbin. — Anand Mukundan
– Anand Mukundan, Commented Apr 6, 2020 at 14:49

Olivia Stork · Accepted Answer · 2020-07-09 17:06:31Z

37

NOTE: If you are using aspnet core 3.0, the solution can be found here. For other versions, keep reading.

You need to mark your parameter as coming from the body with the FromBody attribute like this:

[HttpPost]
[Route("test1")]
[AllowAnonymous]
public IActionResult Test([FromBody] Class1 data)
{
    return Ok();
}

You need to make sure you're using application/json as your content type from Postman:

Resulting in:

Make sure your property setters are public as well:

public class Person
{
    public String Name;
    public Int32 Age;
}

edited Jul 9, 2020 at 17:06

Olivia Stork

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answered Jul 13, 2017 at 16:45

Matt

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5 Comments

Mike Anderson Over a year ago

So that is how I had it originally and the object Class1 was null

Matt Over a year ago

So does it work now? Will edit the answer just to clarify headers.

Chris Pratt Over a year ago

@MikeAnderson: Assuming you have [FromBody], the only other reason data would be null is if none of the posted names matched up with properties on Class1. The modelbinder will only instantiate the class if there's something to instantiate it with (i.e. at least one property on the class has a value).

Mike Anderson Over a year ago

Thanks for clarifying. I see the issue now. The object I was attempting to bind to is an EF generated object. When I created just the class without the EF added constructor and virtual prop, it works like magic lol. So I guess in order for this work in CORE I cannot use persistent models generated by EF, I have to create my own DTO or VM. Which is best practice anyways...thanks!

jmgardn2 Over a year ago

THANK YOU for your bold note on net core 3.0. I've literally been searching for a whole day trying to figure out the issue and nothing else had worked until that.

edencorbin · Accepted Answer · 2019-07-16 15:38:00Z

8

I was struggling with this too, and thought I would share one item that was necessary for me and not in the accepted answer, I needed to add {get; set;} to each attribute in my class as so:

    public class LogString{
        public string val {get; set;}
        public string data {get; set;}
    }

The rest was the same:

        [HttpPost]
        public void Post([FromBody] LogString message)
        {      
            Console.WriteLine(message.val);
        }

After adding that it started working.

answered Jul 16, 2019 at 15:38

edencorbin

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2 Comments

adam_0 Over a year ago

This is the only thing that worked for me. I haven't seen anywhere in the documentation that says you need to use properties instead of member variables! This is frustrating.

dotsa Over a year ago

Agrrr {get; set;} in one class was missing for all properties, wasted an hour! Thanks!

KyleMit · Accepted Answer · 2023-07-11 21:30:11Z

6

I have an ASP.Net Core 3.1 application and for me, none of these solutions worked. I lost a few hairs (and hours of work) until I found the documentation https://learn.microsoft.com/en-us/aspnet/core/web-api/jsonpatch?view=aspnetcore-3.1

In short :

Install the Microsoft.AspNetCore.Mvc.NewtonsoftJson NuGet package.

Update the project's Startup.ConfigureServices method to call AddNewtonsoftJson. For example:

public void ConfigureServices(IServiceCollection services)
{
    services.AddRazorPages()
        .AddNewtonsoftJson();//this is important
}

That's it ! I don't need the [FromBody] attribute.

If it doesn't work, verify that your model's properties have setters (for example public string Message { get; set; }). Don't forget the setter.

edited Jul 11, 2023 at 21:30

KyleMit♦

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answered Jun 19, 2020 at 5:00

Daniel

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Comments

MarredCheese · Accepted Answer · 2021-10-25 21:36:16Z

2

Consider Properties instead of Fields on your binding objects

On .NET 5, I had to add {get; set;} to my public fields to turn them into properties to get them to populate.

Either the model binder in .NET 5 can't work with fields or you have to do something special to make it populate them. I don't know which. It took a few minutes to turn all my fields into properties, so I wasn't going to spend time trying to figure out how to configure it to work with fields.

Note: If you have this problem with request objects, you will probably also have the same issue with response objects.

edited Oct 25, 2021 at 21:36

MarredCheese

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answered May 20, 2021 at 16:17

JamesHoux

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1 Comment

dotsa Over a year ago

Converted to .NET 5 and spent 2 hours understanding why things do not work any more. {get; set;} was a culprit in one class. Thanks.

Alexander Kozachenko · Accepted Answer · 2018-07-18 09:20:36Z

1

I have been struggling with this issue for several hours and none of existing solutions worked for me, but turned out that I had no public parameterless constructor for class of incoming instance. In this case, Class1 had only protected constructors and parameter was always null no matter what. Hope it helps somebody.

answered Jul 18, 2018 at 9:20

Alexander Kozachenko

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Comments

David Lindon · Accepted Answer · 2021-07-28 07:58:59Z

If anyone else is coming to this question and like me you are used to working with .Net Framework MVC you may be creating the JSON object similar to this -

 this.http.post<Stock>("/api/Stock2", { Stock: this.SelectedStock }, this.httpOptions)

and then expecting it to bound to 'Stock'

[HttpPost("Stock2")]
    public JsonResult Stock2(ConsolidatedStock Stock) {
        return this.Json(1);
    }

This will no longer work in .net core, you need to change how you build the JSON -

 this.http.post<Stock>("/api/Stock2", this.SelectedStock, this.httpOptions)

If you want to post multiple objects, you would need to create a new model to hold those objects, for example

public class StockViewModel {
    public ConsolidatedStock Stock { get; set; }
}

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