What is the correct way to POST arrays (Lists) of objects in .NET Core 2?

Question

I have an API method with this signature:

public async Task<IActionResult> PostCompanies([FromBody] List<Company> companies)

...and the auto generated swagger docs show that the JSON should be a plain array:

 [  
       {
          "fuelSiteId":228972,
          "name": "foo"
       },
       {
          "fuelSiteId":300000010,
          "name": "bar"
       }
 ]

However, if I post this back, it doesn't work and I get the error:

"Cannot deserialize the current JSON array (e.g. [1,2,3]) into type 'Company' because the type requires a JSON object"

The fix is to add the array to a dummy property:

{  
    data: [
       {
          "fuelSiteId":228972,
          "name": "foo"
       },
       {
          "fuelSiteId":300000010,
          "name": "bar"
       }
    ]
}

But everything I'm finding on this site implies this shouldn't be necessary and isn't how you'd normally do it in REST. Additionally, it means what the auto-generated docs say isn't actually what should be posted!

Which is wrong? My API code or the auto-gen Swagger docs or something else?

Ideally I would prefer my API to accept the 'plain' version as this seems more standard and more natural.

The correct parameter for your method should be a type that takes an object with a int property and a string property.. FromBody is to handle the post of form data. so if you remove the [FromBody] maybe it works already? The Company type has to have a FuelSiteID (int) and Name (string) property though. — rmjoia
– rmjoia, Commented Nov 9, 2018 at 12:56
@rmjoia My real object has a couple of dozen properties, but I've tried removing FromBody and it doesn't fix the problem. It still needs it assigned to a dummy property to work. — NickG
– NickG, Commented Nov 9, 2018 at 14:12

rmjoia · Accepted Answer · 2018-11-09 15:54:44Z

This also works without having to wrap in another type, give it a try. I've tested with Postman

    [HttpPost]
    public JsonResult InsertPatientAppointment([FromBody] List<Company> companies)
    {
        return new JsonResult(companies.Select(c =>
            new
            {
                c.FuelSiteId,
                c.Name,
            }
        ));
    }

    public class Company
    {
        [JsonProperty("fuelSiteId")]
        public int FuelSiteId
        {
            get;
            set;
        }
        [JsonProperty("name")]
        public string Name
        {
            get;
            set;
        }
    }

You can also check other resource on SO how to post json object array to a web api I had to dig a bit myself as I didn't remember anymore how to do this..

As stated at Parameter Binding in ASP.NET Web API: "To force Web API to read a complex type from the URI, add the [FromUri] attribute to the parameter."

Using [FromBody]

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