ModelState errors are not returned when action contains a ProducesAttribute, but instead produces a 406.

[JoasE]

Describe the bug

When a controller or action contains a ProducesAttribute("text/plain") and an validation error occurs for the model, a 406 is returned instead of a 400.

Might be related to, but I don't think it's a duplicate of: #16889

To Reproduce

https://github.com/JoasE/406

[HttpPost]
[Produces("text/plain")]
public string Get(Test test)
{
    return "It's going to be rainy";
}

public class Test
{
    [Required]
    public string Test2 { get; set; }
}

Execute action:

POST https://localhost:44371/weatherforecast
Accept: */*
Cache-Control: no-cache
Host: localhost:44371
Content-Length: 2

{}

Response: 406

Further technical details

• ASP.NET Core version: 3.1
• Include the output of dotnet --info

.NET Core SDK (reflecting any global.json):
 Version:   3.1.300
 Commit:    b2475c1295

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.18362
 OS Platform: Windows
 RID:         win10-x64
 Base Path:   C:\Program Files\dotnet\sdk\3.1.300\

Host (useful for support):
  Version: 3.1.4
  Commit:  0c2e69caa6

.NET Core SDKs installed:
  2.0.0 [C:\Program Files\dotnet\sdk]
  2.1.202 [C:\Program Files\dotnet\sdk]
  2.1.403 [C:\Program Files\dotnet\sdk]
  2.1.502 [C:\Program Files\dotnet\sdk]
  2.1.504 [C:\Program Files\dotnet\sdk]
  2.1.509 [C:\Program Files\dotnet\sdk]
  2.1.700 [C:\Program Files\dotnet\sdk]
  2.2.104 [C:\Program Files\dotnet\sdk]
  3.1.300 [C:\Program Files\dotnet\sdk]

.NET Core runtimes installed:
  Microsoft.AspNetCore.All 2.1.5 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.1.6 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.1.8 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.1.11 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.1.13 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.1.18 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.2.2 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.2.8 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.App 2.1.5 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.1.6 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.1.8 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.1.11 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.1.13 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.1.18 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.2.2 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.2.8 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.1.4 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 2.0.0 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.0.9 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.6 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.8 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.11 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.13 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.18 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.2.2 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.2.8 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.1.4 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.WindowsDesktop.App 3.1.4 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

To install additional .NET Core runtimes or SDKs:
  https://aka.ms/dotnet-download

• The IDE (VS / VS Code/ VS4Mac) you're running on, and it's version
  VS 16.6.0

[ghost]

Thanks for contacting us.
We're moving this issue to the Next sprint planning milestone for future evaluation / consideration. We will evaluate the request when we are planning the work for the next milestone. To learn more about what to expect next and how this issue will be handled you can read more about our triage process here.

[pranavkm]

Thanks @JoasE. There isn't a built-in formatter that knows how to produce a string representation for a ProblemDetails instance. You could consider configuring InvalidModelStateResponseFactory to produce a string representation of your model state errors if that's what you need to support.

[JoasE]

@pranavkm Many thanks to the team for looking at this and for the quick response. I'm sorry to disturb you once more with this, but I'd like to reiterate a couple of details which in my opinion make this a bug / unexpected behavior, just in case you missed them. Sorry if you've already considered this.

For the 406 Not Acceptable response code the rfc7231 states:

The 406 (Not Acceptable) status code indicates that the target
resource does not have a current representation that would be
acceptable to the user agent, according to the proactive negotiation
header fields received in the request (Section 5.3), and the server
is unwilling to supply a default representation.

The request which is being executed only contains a few headers, 1 of which is a proactive negotiation header of the response content:

POST https://localhost:44371/weatherforecast
Accept: */*
Cache-Control: no-cache
Host: localhost:44371
Content-Length: 2

{}

Note the Accept: */*

Since the request is accepting any media type as response, and no other proactive negotiation headers are specified, I would never expect a server to return a 406 Not Acceptable response code.

Side note:
On top of that I'd like to add that even if the request only accepted text/plain the server may always choose to disregard the header field if it decides that sending a response which doesn't conform to the proactive negotiation is better than sending a 406 Not Acceptable response. It's debatable whether that's the case here.

[pranavkm]

MVC returns a 406 if it is unable to find a formatter that matches the requirement. You could change this by setting https://docs.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.mvc.mvcoptions.returnhttpnotacceptable?view=aspnetcore-3.1.