ViewModel validation for a List

Question

I have the following viewmodel definition

public class AccessRequestViewModel
{
    public Request Request { get; private set; }
    public SelectList Buildi         


        

Answer 1

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 Customers {get;set}

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

Error message:

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