ASP.NET Core Web API & Swagger : getting required field error for IFormFile even when the field is not required

Question

I have a DTO model like this, which I post to the Web API:

public class AddUpdateFundHouseDto : DtoBase
{
    [Required]
    public string ShortName { get; set; }

    [Required]
    public string FullName { get; set; }

    public IFormFile LogoImage { get; set; }
}

I have kept LogoImage non required knowingly to handle the entity update where the logo image is not mandatory but I am still getting model invalid below error about LogoImage when I check it from Swagger.

{
  "type": "https://tools.ietf.org/html/rfc7231#section-6.5.1",
  "title": "One or more validation errors occurred.",
  "status": 400,
  "traceId": "00-a9e895fe7e88261c7d587b083ad70ba3-6f94424b94e7e2c9-00",
  "errors": {
    "LogoImage": [
      "The LogoImage field is required."
    ]
  }
}

It works fine when I pass the file.

So is it not allowed to make IFormFile type non mandatory or something I am missing?

Answer 1

I believe the [Required] attribute is implied by default when the property is not nullable. Try making the property nullable e.g.:

public IFormFile? LogoImage { get; set; }

Answer 2

It works fine when I pass the file. So is it not allowed to make IFormFile type non mandatory or something I am missing?

Well, the bottom line is if you don't specify any annotation or even nullability in that scenario, Non-nullable reference types (like string, int, class objects)are considered implicitly required by ASP.NET Core model binding.

Attempting to bind data without such properties will result in a model validation error, similar to having a [Required] annotation which you are getting now.

Right way to handle:

You can simply put a nullable ? attribute within your property which already been mentioned by the other contributor. So you can either modify as following:

public class AddUpdateFundHouseDto : DtoBase
{
    [Required]
    public string ShortName { get; set; }

    [Required]
    public string FullName { get; set; }

    public IFormFile? LogoImage { get; set; }
}

Swagger annotations:

You can also use swagger annotations in order to allow IFormFile null property in request.

public class AddUpdateFundHouseDto 
{
    [Required]
    public string ShortName { get; set; }

    [Required]
    public string FullName { get; set; }

    [SwaggerParameter(Required = false)]
    public IFormFile? LogoImage { get; set; }
}

Output:

Note: Please refer to this official document if you want to study more.