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"); 

Answer 2

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; }