public enum ProductQuantityType {
    Weight = 1,
    Volume = 2,
    Custom = 0
}
This fails
[MetadataType(typeof(ProductMetaData))]
public partial class Product {
    public class ProductMetaData {
        [DefaultValue(ProductQuantityType.Weight)]
        public object QuantityType { get; set; }
    }
}
Error: An object reference is required for the non-static field, method, or property