JSON "null" request body rejected as an empty request body

Is there an existing issue for this?

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Describe the bug

I have an action method which accepts a posted null value for the body:

		[HttpPost("acceptsNull")]
		public string Upload(WeatherForecast? forecast = null)
		{
			return JsonConvert.SerializeObject(forecast);
		}

However, when I post a JSON null to this endpoint I get a 400 response complaining that the body is empty.

Expected Behavior

Since the body is not empty and JSON-deserializes to the model parameter type just fine, I expect it to invoke the action with a null model.

If it can’t do that, I at least expect an error message that indicates nulls are not permitted rather than one claiming the body is empty (although if the body were actually empty instead of valid JSON I’d probably want this request to be rejected).

Steps To Reproduce

Call the above endpoint with:

using var client = new HttpClient();
using var response = client.PostAsJsonAsync(".../acceptsNull", default(WeatherForecast));
Console.WriteLine(response.StatusCode); // 400
Console.WriteLine(await response.Content.ReadAsStringAsync()); //  ...A non-empty request body is required....

Exceptions (if any)

No response

.NET Version

6.0.100

Anything else?

The workaround I found is to set [FromBody(EmptyBodyBehavior = EmptyBodyBehavior.Allow)], but I don’t really want to have to do this because I’d like to reject empty bodies and accept non-empty bodies which are JSON null.

[senbar]

What’s the error message I should put in? Is something like “Null is not permitted in request body” fine? Edit: I have opened pr draft to decide on what message should look like

[rjgotten]

A difference here is that query strings do not have a native encoding for null

Technically they don’t have a native encoding at all. Everything after the ? save for the # and whatever meaning a server wants to attribute to it, is fair game. However, there are some particular schemes which grew into ad-hoc quote-unquote ‘standards’ such as the way the ASP.NET query string parsers and model binder work; the way jQuery’s serialization of objects to query strings works (which iirc is a derivative of how PHP parses query strings?)

One convention for representing null query string values, or rather: a value-less key - is simply ?key. Contrast with ?key= which represents the empty string.

The former is not supported entirely as such by ASP.NET, afaik. I think all the parsing methods the framework supports, either drop the key entirely or rather than a value-less key turn it into a key-less value - i.e. a null key with a value of "key".

(Well; at least it’s consistently inconsistent with the rest of the world 🤷‍♂️ )