ApiExplorer handles [FromHeader] binding (on complex type) differently to [FromQuery] binding

Describe the bug

Given the following two actions that bind query/header params to a complex type:

public IActionResult Get1([FromQuery]CustomParams queryParams)

public IActionResult Get2([FromHeader]CustomParams headerParams)

public class CustomParams
{
    public string Foo { get; set; }
    public string Bar { get; set; }
}

For the first action, ApiExplorer surfaces multiple query parameters, one for each property in the complex type. This makes sense because that’s exactly how the model binder behaves - i.e. it will look for individual query parameters and assign them to corresponding properties. However, for the second action, ApiExplorer only surfaces a single parameter corresponding to the custom type itself. It’s my understanding that the model binder behaves the same way in both cases, and so I would expect ApiExplorer to follow suit. Here’s an example of the relevant ApiExplorer data (serialized as JSON):

[
  {
    "relativePath": "Test/Get1",
    "httpMethod": "GET",
    "parameters": [
      {
        "name": "Foo",
        "source": {
          "displayName": "Query",
          "id": "Query",
          "isGreedy": false,
          "isFromRequest": true
        },
        "modelType": "System.String"
      },
      {
        "name": "Bar",
        "source": {
          "displayName": "Query",
          "id": "Query",
          "isGreedy": false,
          "isFromRequest": true
        },
        "modelType": "System.String"
      }
    ]
  },
  {
    "relativePath": "Test/Get2",
    "httpMethod": "GET",
    "parameters": [
      {
        "name": "headerParams",
        "source": {
          "displayName": "Header",
          "id": "Header",
          "isGreedy": true,
          "isFromRequest": true
        },
        "modelType": "ApiExplorerIssue.Controllers.CustomParams"
      }
    ]
  }
]

To Reproduce

> git clone [e-mail]:domaindrivendev/ApiExplorerIssue.git
> cd ApiExplorerIssue
> git checkout from-header-property-bindings
> dotnet run

Then navigate to "http://localhost:5000/apidescriptions" to see the ApiExplorer data

Further technical details

• ASP.NET Core version: 5.0

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[Thepriestdude]

This becomes a whole other beast when working with Commands (CQRS) and utilizing ‘.AddEndpointFilter’ for the command, since the headers are never picked up if using a complex type:

public sealed record Command(
   [FromHeader] string HeaderKey, 
   [FromBody] string BodyKey
);

public class CommandValidator : AbstractValidator<Command>
{
    public CommandValidator()
    {
        // validation for header & body
        // ...
    }
}

[...]

public static void AddCommandEndpoints(this IEndpointRouteBuilder app)
{
   app.MapPost("/postCommand", (Command command) => {[...]})
     .AddEndpointFilter([...]) // pass command to CommandValidator
}

This will cause the validation to happen before entering the endpoint, thereby causing us never to be able to validate the header.

A workaround for this is to accept the ‘headerKey’ and ‘bodyKey’ as separate parameters, map them to the command, and then validate them. But this seems like something that is not intentional.