ValidationAttribute for IEnumerable Property

Question

I have this custom validation attribute for validating a collection. I need to adapt this to work with IEnumerable. I tried making the attribute a generic, but you can't have a generic attribute.

[AttributeUsage(AttributeTargets.Property, AllowMultiple = false, Inherited = true)]
public class CollectionHasElements : System.ComponentModel.DataAnnotations.ValidationAttribute 
{
     public override bool IsValid(object value)
     {
         if (value != null && value is IList)
         {
             return ((IList)value).Count > 0;
         }
         return false;
     }
}

I'm having trouble casting it to an IEnumerable so that I can check its count() or any().

Any ideas?

Answer 1

Try this

var collection = value as ICollection;
if (collection != null) {
    return collection.Count > 0;
}

var enumerable = value as IEnumerable;
if (enumerable != null) {
    return enumerable.GetEnumerator().MoveNext();
}


return false;

or, since C# 7.0 with pattern matching:

if (value is ICollection collection) {
    return collection.Count > 0;
}
if (value is IEnumerable enumerable) {
    return enumerable.GetEnumerator().MoveNext();
}
return false;

Note: Testing for ICollection.Count is more efficient than getting an enumerator and beginning to enumerate the enumerator. Therefore I try to use the Count property whenever possible. However, the second test would work alone, since a collection always implements IEnumerable.

The inheritance hierarchy goes like this: IEnumerable > ICollection > IList. IList implements ICollection and ICollection implements IEnumerable. Therefore IEnumerable will work for any well designed type of collection or enumeration but not IList. For instance Dictionary<K,V> does not implement IList but ICollection and therefore also IEnumeration.

---

The .NET naming conventions say that an attribute class name should always end in "Attribute". Therefore your class should be named CollectionHasElementsAttribute. When applying the attribute you can drop the "Attribute" part.

[CollectionHasElements]
public List<string> Names { get; set; }