ViewModel validation for a List

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");

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");

Following code works in asp.net core 1.1.

[Required, MinLength(1, ErrorMessage = "At least one item required in work order")]
public ICollection<WorkOrderItem> Items { get; set; }

Another possible way to handle the count validations for view model object's collection members, is to have a calculated property returning the collection or list count. A RangeAttribute can then be applied like in the code below to enforce count validation:

[Range(minimum: 1, maximum: Int32.MaxValue, ErrorMessage = "At least one item needs to be selected")]
public int ItemCount
{
    get
    {
        return Items != null ? Items.Length : 0;
    }
}

In the code above, ItemCount is an example calculated property on a view model being validated, and Items is an example member collection property whose count is being checked. In this example, at least one item is enforced on the collection member and the maximum limit is the maximum value an integer can take, which is, for most of the practical purposes, unbounded. The error message on validation failure can also be set through the RangeAttribute's ErrorMessage member in the example above.

Darin's answer is good but the version below will automatically give you a useful error message.

public class MinimumElementsAttribute : ValidationAttribute
{
    private readonly int minElements;

    public MinimumElementsAttribute(int minElements)
    {
        this.minElements = minElements;
    }

    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        var list = value as IList;

        var result = list?.Count >= minElements;

        return result
            ? ValidationResult.Success
            : new ValidationResult($"{validationContext.DisplayName} requires at least {minElements} element" + (minElements > 1 ? "s" : string.Empty));
    }
}

Usage:

[MinimumElements(1)]
public List<Customer> Customers {get;set}

[MinimumElements(2)]
public List<Address> Addresses {get;set}

Error message:

Customers requires at least 1 element
Addresses requires at least 2 elements

ViewModel validation for a List

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