Require not null collection elements in when validating model in aspnet core

Question

I have example mode class:

public class Data
{
        [Required]
        [MinLength(1)]
        public List<Foo> Foos{ get; set; }
}

which is passed to controller:

[ApiController]
public class FooController : ControllerBase
{
  public async Task<IActionResult> Post([FromBody] Data data)
  {
      // Do stuff
  }
}

I have nullable annotations enabled in my project configuration.

How can I force model validator to reject request with null foos array? For example following request is accepted by controller:

{
   foos: [null]
}

I would like such request to return bad request status code.

Answer 1

When using Data Annotations the child classes are not validated. I recommend Fluent Validation for the use case presented.

The use is simple, just define a rule

public class DataValidator : AbstractValidator<Data>
{
    public DataValidator()
    {
        // RuleFor(r => r.Foos).NotEmpty(); // <-- not needed as Nullable Annotations are enabled
        RuleForEach(r => r.Foos).NotNull();
    }
}

And register FluentValidation in configure services method

services.AddControllers()
            .AddFluentValidation(configuration => configuration.RegisterValidatorsFromAssemblyContaining<Startup>());

Answer 2

You could custom ValidationAttribute to custom the rule:

Custom ValidationAttribute:

public class ValidArray : ValidationAttribute
{
    protected override ValidationResult
            IsValid(object value, ValidationContext validationContext)
    {
        var model = (Data)validationContext.ObjectInstance;
        if(Extension.IsNullOrEmpty<Foo>(model.Foos))
        {
            return new ValidationResult
                ("Array cannot be null");
        }
        else
        {
            return ValidationResult.Success;
        }
    }
}

Custom IsNullOrEmpty :

public static class Extension
{
    public static bool IsNullOrEmpty<T>(List<T> array) where T : class
    {
        if (array == null || array.Count == 0)
            return true;
        else
            return array.All(item => item == null);
    }
}

Model:

public class Data
{
    //[Required]
    [MinLength(1)]
    [ValidArray]
    public List<Foo> Foos { get; set; }
}
public class Foo
{
    public int Id { get; set; }
    public int Count { get; set; }
}

Result: