Can I use DataAnnotations to validate a collection property?

Question

I have a model for a WebAPI2 controller with a field that takes in a collection (List) of strings. Is there a way that I can specify DataAnnotations (eg [MaxLength]) for the strings to ensure, via validation, that none of the strings in the list is > 50 in length?

    public class MyModel
    {
        //...

        [Required]
        public List<string> Identifiers { get; set; }

        // ....
    }

I'd rather not create a new class simply to wrap the string.

Answer 1

You can write your own validation attribute, eg as follows:

public class NoStringInListBiggerThanAttribute : ValidationAttribute
{
    private readonly int length;

    public NoStringInListBiggerThanAttribute(int length)
    {
        this.length = length;
    }

    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        var strings = value as IEnumerable<string>;
        if(strings == null)
            return ValidationResult.Success;

        var invalid = strings.Where(s => s.Length > length).ToArray();
        if(invalid.Length > 0)
            return new ValidationResult("The following strings exceed the value: " + string.Join(", ", invalid));

        return ValidationResult.Success;
    }
}

You will be able to place it directly over your property:

[Required, NoStringInListBiggerThan(50)]
public List<string> Identifiers {get; set;}

Answer 2

I know this question is ages old, but for those that stumble upon it, here is my version of a custom attribute inspired by the accepted answer. It makes use of the StringLengthAttribute to do the heavy lifting.

/// <summary>
///     Validation attribute to assert that the members of an IEnumerable&lt;string&gt; property, field, or parameter does not exceed a maximum length
/// </summary>
[AttributeUsage(AttributeTargets.Property | AttributeTargets.Field | AttributeTargets.Parameter)]
public class EnumerableStringLengthAttribute : StringLengthAttribute
{
    /// <summary>
    ///     Constructor that accepts the maximum length of the string.
    /// </summary>
    /// <param name="maximumLength">The maximum length, inclusive.  It may not be negative.</param>
    public EnumerableStringLengthAttribute(int maximumLength) : base(maximumLength)
    {
    }
    
    /// <summary>
    ///     Override of <see cref="StringLengthAttribute.IsValid(object)" />
    /// </summary>
    /// <remarks>
    ///     This method returns <c>true</c> if the <paramref name="value" /> is null.
    ///     It is assumed the <see cref="RequiredAttribute" /> is used if the value may not be null.
    /// </remarks>
    /// <param name="value">The value to test.</param>
    /// <returns><c>true</c> if the value is null or the length of each member of the value is between the set minimum and maximum length</returns>
    /// <exception cref="InvalidOperationException"> is thrown if the current attribute is ill-formed.</exception>
    public override bool IsValid(object? value)
    {
        return value is null || ((IEnumerable<string>)value).All(s => base.IsValid(s));
    }
}

Edit: base.IsValid(s) returns true if s is null, so if the string members must not be null, use this instead:

public override bool IsValid(object? value)
{
    return value is null || ((IEnumerable<string>)value).All(s => !string.IsNullOrEmpty(s) && base.IsValid(s));
}