ViewModel validation for a List

Question

I have the following viewmodel definition

public class AccessRequestViewModel
{
    public Request Request { get; private set; }
    public SelectList Buildings { get; private set; }
    public List<Person> Persons { get; private set; }
}

So in my application there must be at least 1 person for an access request. What approach might you use to validate? I don't want this validation to happen in my controller which would be simple to do. Is the only choice a custom validation attribute?

Edit: Currently performing this validation with FluentValidation (nice library!)

RuleFor(vm => vm.Persons)
                .Must((vm, person) => person.Count > 0)
                .WithMessage("At least one person is required");

Answer 1

If you are using Data Annotations to perform validation you might need a custom attribute:

public class EnsureOneElementAttribute : ValidationAttribute
{
    public override bool IsValid(object value)
    {
        var list = value as IList;
        if (list != null)
        {
            return list.Count > 0;
        }
        return false;
    }
}

and then:

[EnsureOneElement(ErrorMessage = "At least a person is required")]
public List<Person> Persons { get; private set; }

or to make it more generic:

public class EnsureMinimumElementsAttribute : ValidationAttribute
{
    private readonly int _minElements;
    public EnsureMinimumElementsAttribute(int minElements)
    {
        _minElements = minElements;
    }

    public override bool IsValid(object value)
    {
        var list = value as IList;
        if (list != null)
        {
            return list.Count >= _minElements;
        }
        return false;
    }
}

and then:

[EnsureMinimumElements(1, ErrorMessage = "At least a person is required")]
public List<Person> Persons { get; private set; }

Personally I use FluentValidation.NET instead of Data Annotations to perform validation because I prefer the imperative validation logic instead of the declarative. I think it is more powerful. So my validation rule would simply look like this:

RuleFor(x => x.Persons)
    .Must(x => x.Count > 0)
    .WithMessage("At least a person is required");