ASP.NET Core Web API endpoint validating nullable property

Question

In my ASP.NET Core 6 Web API project, I have a FolderDto type that has a nullable ParentFolder property:

public class FolderDto
{
    public string Name { get; set; } = string.Empty;
    public string Description { get; set; } = string.Empty;

    public Folder? ParentFolder { get; set; }
}

I also have an endpoint that receives a FolderDto:

[HttpPost]
public async Task<Folder> Post([FromBody] FolderDto folderDto)
{
    var folder = mapper.Map<Folder>(folderDto);
    folder.FolderOwner = folderDto.ParentFolder?.FolderOwner ?? await userService.FindByIdAsync(1);
    folderService.CreateFolder(folder);
    return folder;
}

When testing this endpoint in Swagger, I get the following error:

"errors": { "ParentFolder.FolderOwner": [ "The FolderOwner field is required." ] }

When testing the endpoint in Swagger, I am intentionally setting the ParentFolder as an empty object:

{
  "name": "string",
  "description": "string",
  "parentFolder": { }
}

Why is .NET trying to validate a property of the ParentFolder property when I have set it to be nullable?

Answer 1

You made parentFolder nullable - which means you can leave out a value for parentFolder and omit that element from your JSON entirely - or use null.

But you DID NOT make the actual object it refers to (the parentFolder's class) all nullable - which means, you CANNOT provide an empty object in your JSON - unless you've made ALL properties of the parentFolder class nullable.

In short: an empty object is NOT the same thing as or equivalent to null. It's still an object (with all its properties empty/default/null) - but it's an object - not null which is no object / nothing / absence of any value