ASP.NET core - how to pass optional [FromBody] parameter?

user_3934322 0
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How do I pass optional (nullable) [FromBody] parameter in ASP.NET Core (5.0)? If I don't send body in my request I get 415 Unsupported Media Type error. Can this be configured and if so, how to do it on a controller or action, rather than an app level? I presume it has to do something with model validation, but not sure. Thanks.

[HttpGet("[action]")]
public async Task<IActionResult> GetElementsAsync([FromBody] IEnumerable<int> elements = default)
{
  var result = await dataService.GetData(elements);
  return Ok(result);
}

EDIT: To clarify:

This is typical scenario and it works normally: This is typical scenario and it works normally

But passing empty body is returning 415 right away without even reaching action: Passing empty body is returning 415 right away without reaching action

asp.net-core controller
4 Answers
user_111099670

Just add content-type in your request header. Without the content-type:application/json will appear 415 when body is empty.

No changes to your controller. Test in my side is ok.

enter image description here

I created a new asp.net core 5 api project and this is my controller:

using Microsoft.AspNetCore.Mvc;
using System.Collections.Generic;

namespace WebApi_net5.Controllers
{
    public class HomeController : ControllerBase
    {
        [HttpGet("[action]")]
        public string GetElementsAsync([FromBody] IEnumerable<int> elements = default)
        {
            return "value";
        }
    }
}
user_155812270

I will address some points that were not mentioned here.

  1. To get rid of 415 without sending Content-Type you need to create your custom consumer of the Content-Type

https://stackoverflow.com/a/65813534/2531209

But I would say this is an overkill

  1. If you pass Content-Type: application/json in your request header you will get "Body cannot be empty" (Tested on .NET 6) and only then @Nicola answer comes in handy.

From my tests it looks like modifications to the controller are not needed and only FromBody(EmptyBodyBehavior = EmptyBodyBehavior.Allow) is enough with nullable type as parameter.

  1. Or you can change nothing in your current code and send a header Content-Type: application/json with a body of {}. This will bypasses all of those errors, but this is not the most optimal solutions for public API's
user_25312090

With ASP.NET Core 3.1, I could allow nullable optional parameters by implementing Nicola's suggestion:

services.AddControllers(options =>{options.AllowEmptyInputInBodyModelBinding = true;})
user_102792580
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