How to receive an array of objects (with IFormFile) using [FromForm] in ASP.NET Core Web API?

Question

I'm building an ASP.NET Core Web API endpoint that needs to receive an array of complex objects via multipart/form-data. Each object includes key-value string pairs and an optional file upload. Each FormItem must have either a value or a file, but not both, and not neither. Exactly one must be provided.

Here's my model:

public class FormItem
{
    public string key { get; set; }
    public string value { get; set; }
    public IFormFile file { get; set; }
}

And my controller action looks like this:

[HttpPost]
public async Task<ActionResult> SaveAdditionalInfo(
    [FromForm] FormItem[] formFields,
    [FromRoute] Guid businessKycId,
    [FromQuery] string token)
{
...
}

❓ What I want:

• To receive an array of FormItem via a multipart/form-data POST request.
• To support both key-value pairs and file uploads.
• To support it in Swagger UI (using Swashbuckle) as well.

❌ Problem:

• Model binding does not populate the formFields correctly when submitted from Swagger or Postman.
• Swagger UI doesn't render this correctly either — no way to add multiple FormItem entries with files.

How can I properly receive an array of complex objects (with IFormFile) from a form post in ASP.NET Core?

Is there a recommended way to make this work with both model binding and Swagger UI?

Answer 1

Change formFields to be IEnumerable<FormItem>. The default router and json parser doesn't see a json array as a .NET array.