.NET 5.0 Web API won't work with record featuring required properties - asp.net-core-webapi

Question

I'm using a C# 9.0 record type as a binding model for a .NET 5.0 Web API project. Some of the properties are required.
I'm using the record positional syntax, but am receiving errors.
public record Mail(
System.Guid? Id,
[property: Required]
string From,
[property: Required]
string[] Tos,
[property: Required]
string Subject,
string[]? Ccs,
string[]? Bccs,
[property: Required]
Content[] Contents,
Attachment[]? Attachments
);
This is then exposed as the binding model for my Index action:
public async Task<ActionResult> Index(Service.Models.Mail mailRequest)
{
…
}
Whenever I try to make a request, however, I receive the following error:
Record type 'Service.Models.Mail' has validation metadata defined on property 'Contents' that will be ignored. 'Contents' is a parameter in the record primary constructor and validation metadata must be associated with the constructor parameter.
I tried removing the attribute on the Contents property, but it then fails for the next (prior) property. I tried using [param: …] instead of [property: …], as well as mixing them, but keep getting the same kind of error.
I looked around the web, and haven't found any suggestion of handling annotations differently for C# 9 records. I did my best, but I'm out of ideas—outside of converting my records to POCOs.

Answer 1

I gave up using Positional constructor, and with the verbose full declaration of the properties, it works.
public record Mail
{
public System.Guid? Id { get; init; }
[Required]
public string From { get; init; }
[Required]
public string[] Tos { get; init; }
[Required]
public string Subject { get; init; }
public string[]? Ccs { get; init; }
public string[]? Bccs { get; init; }
[Required]
public Content[] Contents { get; init; }
public Attachment[]? Attachments { get; init; }
public Status? Status { get; init; }
public Mail(Guid? id, string #from, string[] tos, string subject, string[]? ccs, string[]? bccs, Content[] contents, Attachment[]? attachments, Status status)
{
Id = id;
From = #from;
Tos = tos;
Subject = subject;
Ccs = ccs;
Bccs = bccs;
Contents = contents;
Attachments = attachments;
Status = status;
}
}

Answer 2

Try using only [Required] (instead of [property: Required]), for some reason worked for me

Answer 3

For me it started to work by adding the [ApiController] attribute to the controller.

Answer 4

I found something similar on ASP.NET Core Razor pages getting:
InvalidOperationException: Record type 'WebApplication1.Pages.LoginModelNRTB+InputModel' has validation metadata defined on property 'PasswordB' that will be ignored. 'PasswordB' is a parameter in the record primary constructor and validation metadata must be associated with the constructor parameter.
from
Microsoft.AspNetCore.Mvc.ModelBinding.ModelMetadata.ThrowIfRecordTypeHasValidationOnProperties()
After some digging, I found: https://github.com/dotnet/aspnetcore/blob/main/src/Mvc/Mvc.Core/src/ModelBinding/Validation/DefaultComplexObjectValidationStrategy.cs
So maybe as you've done, the verbose declaration is the way forward.
Positional record attributes in ASP.NET Core background
How do I target attributes for a record class? more background

Answer 5

Using FluentValidation and keeping properties with the full declaration seems to work perfectly in my case. I highly recommend trying this highly polished alternative validation library instead of using the pretty old standard data annotations
public record LoginViewModel
{
public string Mail { get; init; }
public string Password { get; init; }
public bool IsPersistent { get; init; }
}
public class LoginValidator : AbstractValidator<LoginViewModel>
{
public LoginValidator()
{
RuleFor(l => l.Mail).NotEmpty().EmailAddress();
RuleFor(l => l.Password).NotEmpty();
}
}