How do you apply validation attributes to objects in collections?

Question

Basically, if I have a collection of objects, how can I apply a validation attribute to each item in the collection (such as MaxLengthAttribute)?

public class Foo
{
    public ICollection<string> Bars { get; set; }
}

For example, how can I ensure that Bars contains strings that validate against a max length of 256?

Update:

I understand how to apply a validation attribute on a single property, but the question is asking how to apply it on objects within a collection.

public class Foo
{
    [StringLength(256)] // This is obvious
    public string Bar { get; set; }

    // How do you apply the necessary attribute to each object in the collection!
    public ICollection<string> Bars { get; set; }
}

Answer 1

I know this question is kinda old, but maybe someone comes along looking for answers.

I'm not aware of a generic way to apply attributes to collection items, but for the specific string length example I used the following:

public class StringEnumerationLengthValidationAttribute : StringLengthAttribute
{
    public StringEnumerationLengthValidationAttribute(int maximumLength)
        : base(maximumLength)
    { }

    public override bool RequiresValidationContext { get { return true; } }
    public override bool IsValid(object value)
    { return false; }

    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        var e1 = value as IEnumerable<string>;
        if (e1 != null) return IsEnumerationValid(e1, validationContext);
        return ValidationResult.Success; // what if applied to something else than IEnumerable<string> or it is null?
    }

    protected ValidationResult IsEnumerationValid(IEnumerable<string> coll, ValidationContext validationContext)
    {
        foreach (var item in coll)
        {
            // utilize the actual StringLengthAttribute to validate the items
            if (!base.IsValid(item) || (MinimumLength > 0 && item == null))
            {
                return new ValidationResult(base.FormatErrorMessage(validationContext.DisplayName));
            }
        }
        return ValidationResult.Success;
    }
}

Apply as follows, to require 4-10 characters for each collection item:

[StringEnumerationLengthValidation(10, MinimumLength=4)]
public ICollection<string> Sample { get; set; }

Answer 2

OK I found a pretty nice article explaining some useful info about this:

http://blogs.msdn.com/b/codeanalysis/archive/2006/04/27/faq-why-does-donotexposegenericlists-recommend-that-i-expose-collection-lt-t-gt-instead-of-list-lt-t-gt-david-kean.aspx

Here is some suggested code that would make the Bars member of Foo do what you want.

public class Foo
{
    public ValidatedStringCollection Bars = new ValidatedStringCollection(10);
}

public class ValidatedStringCollection : Collection<string>
{

    int _maxStringLength;

    public ValidatedStringCollection(int MaxStringLength)
    {
        _maxStringLength = MaxStringLength;
    }

    protected override void InsertItem(int index, string item)
    {
        if (item.Length > _maxStringLength)
        {
            throw new ArgumentException(String.Format("Length of string \"{0}\" is beyond the maximum of {1}.", item, _maxStringLength));
        }
        base.InsertItem(index, item);
    }

}

class Program
{
    static void Main(string[] args)
    {
        Foo x = new Foo();
        x.Bars.Add("A");
        x.Bars.Add("CCCCCDDDDD");
        //x.Bars.Add("This string is longer than 10 and will throw an exception if uncommented.");

        foreach (string item in x.Bars)
        {
            Console.WriteLine(item);
        }

        Console.ReadKey();
    }
}

The linked article has several suggestions including overriding the other methods on the collection, implementing events conditionally, etc. This should hopefully cover you.