How to send nullable DateTime in form-data / [FromForm]

Question

I'm trying to make an HTTP request to my controller and send back a nullable DateTime. This request succeeds when the field is populated, when the field (CloseDate) is null, I get back an HTTP response saying null is not valid (for Close Date).

In my controller I have a post method that looks likes this:

    [HttpPost]
    [Authorize]
    [Route("properties/{propertyId}")]
    public async Task<IActionResult> BasicPropertyUpdate([FromForm] PropertyViewModel request)
    {

    }

Since I am getting a HTTP 400 response I don't get to hit any breakpoints.

In my ViewModel I have a contract that looks like this:

    using System;
    using System.Collections.Generic;
    using System.ComponentModel.DataAnnotations;
    using System.Linq;
    using System.Threading.Tasks;

    namespace ViewModels
    {
        public class PropertyViewModel
        {
            //other stuff
            public DateTime? CloseDate { get; set; }
        }
    }

The raw HTTP response (as captured by Fiddler) looks like this:

    HTTP/1.1 400 Bad Request
    Date: Tue, 10 Dec 2019 22:07:01 GMT
    Content-Type: application/json; charset=utf-8
    Server: Kestrel
    Vary: Origin
    Access-Control-Allow-Credentials: true
    Access-Control-Allow-Origin: http://localhost:3000
    Access-Control-Expose-Headers: TotalCount
    Request-Context: appId=cid-v1:712f8d3d-6af6-44c6-a573-2b9adda915c7
    Content-Length: 62

    {"closeDate":["The value 'null' is not valid for CloseDate."]}

The form-data in my HTTP Request (as captured by Fiddler) looks like this:

    -----------------------------32591187621655
    Content-Disposition: form-data; name="closeDate"

    null

Is there some kind of inherent limitation to [FromForm] that prevents sending of nullable DateTime fields? If so, is there some recommended alternative that gets the job done?

Answer 1

"null" is a JSON literal but a JSON deserializer isn't used to read the form data. It's being posted as a string of 4 characters.

The model binder will bind an empty string as null to a nullable value type such as DateTime?. If you were POST-ing the data as application/x-www-form-urlencoded, you could omit the name/value pair or specify an empty string (i.e. closeDate=).

This form data appears to be encoded as mime/multipart but the same principle holds. A zero-length mime part is an empty string, which the model binder can then interpret as null.