How does binding source parameter inference work?

[Robar666]

Is this a Bug or Feature request?:

Question, I've also created a SO question.

Steps to reproduce (preferably a link to a GitHub repo with a repro project):

• Create ApiController (see code below)
• Make an Ajax request (I used Postman)
  • URL: http://localhost:5000/account/backendLogin
  • Method: POST
  • ContentType: application/x-www-form-urlencoded
  • Body: email=asdf%40asdf.com&password=asdf

// AccountController.cs

[Route("/account"), ApiController]
public class AccountController : ControllerBase {
    [HttpPost("backendLogin"), AllowAnonymous]
    public async Task<ActionResult<LoginResponse>> BackendLogin(LoginRequest lr) {
        await Task.CompletedTask.ConfigureAwait(false); // do some business logic
        return Ok(new LoginResponse {UserId = "123"});
    }

    // Models

    public class LoginRequest {
        public string Email { get; set; }
        public string Password { get; set; }
    }

    public class LoginResponse {
        public string UserId { get; set; }
    }
}

// Startup.cs

public void ConfigureServices(IServiceCollection services) {
    services.AddMvcCore()
                    .AddJsonFormatters(settings => {
                        settings.DateTimeZoneHandling = DateTimeZoneHandling.Utc;
                        settings.DateFormatHandling = DateFormatHandling.IsoDateFormat;
                        settings.ContractResolver = new CamelCasePropertyNamesContractResolver();
                    })
                    .SetCompatibilityVersion(CompatibilityVersion.Version_2_1);

    services.Configure<ApiBehaviorOptions>(options => {
        // options.SuppressConsumesConstraintForFormFileParameters = true;
        // options.SuppressInferBindingSourcesForParameters = true;
        // options.SuppressModelStateInvalidFilter = true;
    });
}

public void Configure(IApplicationBuilder app, IHostingEnvironment env) {
    if (env.IsDevelopment()) {
        app.UseDeveloperExceptionPage();
    }
    app.UseMvc();
}

Description of the problem:

It seems that the binding source parameter inference isn't working as expected (at least for me). I assumed based on the docs, that complex types (e.g. my LoginRequest) are inferred as [FromBody].

If I change the options.SuppressInferBindingSourcesForParameters in ConfigureServices to true, it seems to work. This is strange, since this setting should disable the binding inference or have I misconceived something? Is this a bug in ASP.NET core or am I missing something?

Btw. it also works if I ommit the ApiController attribute, but I guess this is not the real solution to this problem

PS: I would also suggest to update the documentation for ApiBehaviorOptions.SuppressInferBindingSourcesForParameters. Maybe it is just me, but it was not clear on first sight what this property does based on the documentation. The second paragraph starts with When enabled, ..., I first thought it means when this property is set to true, but I think it really means when the "Binding inference" is enabled.

Version of Microsoft.AspNetCore.Mvc or Microsoft.AspNetCore.App or Microsoft.AspNetCore.All: 2.1.1

[pranavkm]

ApiController is designed for REST-client specific scenarios and isn't designed towards browser based (form-urlencoded) requests. FromBody assumes JSON \ XML request bodies and it'll attempt to serialize it, which is not what you want with form url encoded content. Using a vanilla (non-ApiController) would be the way to go here.

The second paragraph starts with When enabled, ..., I first thought it means when this property is set to true, but I think it really means when the "Binding inference" is enabled.

It does mean when the property is set to true \ when the feature is enabled. We'd be happy to accept PRs to update it if you think it could be worded better.

[Robar666]

Thank you for the explanation.
I will go for the vanilla controller for now and maybe later change the client calls for JSON bodies.