Data validation with custom attributes (AttributeTargets.Class) on EF buddy classes

Question

I have an Entity Framework generated class with the following properties:

public DateTime LaunchDate;
public DateTime ExpirationDate;

I nee

Answer 1

Got it to work by implementing TypeID. I assigned the attribute to the partial class:

    [MetadataType(typeof(PromotionValidation))]
    [PropertyMustBeGreaterThanAttribute("RetailPrice", "DiscountedPrice")]
    [PropertyMustBeGreaterThanAttribute("ExpirationDate", "LaunchDate")]
    [PropertyMustBeGreaterThanAttribute("ClaimDeadline", "ExpirationDate")]
    public partial class Promotion
    {
... 

Here is the listing in case anyone needs it:

[AttributeUsage(AttributeTargets.Class, AllowMultiple = true, Inherited = true)]
public class PropertyMustBeGreaterThanAttribute : ValidationAttribute
{
    private const string _defaultErrorMessage = "'{1}' must be greater than '{0}'!";
    private readonly object _typeId = new object();

    public PropertyMustBeGreaterThanAttribute(string property, string greaterThan)
        : base(_defaultErrorMessage)
    {
        GreaterThan = greaterThan;
        Property = property;
    }

    public string Property { get; private set; }
    public string GreaterThan { get; private set; }

    public override string FormatErrorMessage(string name)
    {
        return String.Format(CultureInfo.CurrentUICulture, ErrorMessageString,
            GreaterThan, Property);
    }

    public override bool IsValid(object value)
    {
        PropertyDescriptorCollection properties = TypeDescriptor.GetProperties(value);
        IComparable greaterThan = properties.Find(GreaterThan, true /* ignoreCase */).GetValue(value) as IComparable;
        IComparable property = properties.Find(Property, true /* ignoreCase */).GetValue(value) as IComparable;
        return greaterThan.CompareTo(property) > 0;
    }

    public override object TypeId
    {
        get
        {
            return _typeId;
        }
    }
}