Conditionally validating portions of an ASP.NET MVC Model with DataAnnotations?

Question

I have certain panels on my page that are hidden under certain circumstances.

For instance I might have a \'billing address\' and \'shipping address\' and I dont wan

Answer 1

For the CheckoutModel I am using this approach (most fields hidden):

[ModelBinder(typeof(CheckoutModelBinder))]
public class CheckoutModel : ShoppingCartModel
{        
    public Address BillingAddress { get; set; }
    public Address ShippingAddress { get; set; }
    public bool ShipToBillingAddress { get; set; }
}

public class Address
{
    [Required(ErrorMessage = "Email is required")]
    public string Email { get; set; }

    [Required(ErrorMessage = "First name is required")]
    public string FirstName { get; set; }

    [Required()]
    public string LastName { get; set; }

    [Required()]
    public string Address1 { get; set; }
}

The custom model binder removes all ModelState errors for fields beginning with 'ShippingAddress' if it finds any. Then 'TryUpdateModel()' will return true.

    public class CheckoutModelBinder : DefaultModelBinder
    {
        protected override void OnModelUpdated(ControllerContext controllerContext,
                                               ModelBindingContext bindingContext) {

            base.OnModelUpdated(controllerContext, bindingContext);

            var model = (CheckoutModel)bindingContext.Model;

            // if user specified Shipping and Billing are the same then 
            // remove all ModelState errors for ShippingAddress
            if (model.ShipToBillingAddress)
            {
                var keys = bindingContext.ModelState.Where(x => x.Key.StartsWith("ShippingAddress")).Select(x => x.Key).ToList();
                foreach (var key in keys)
                {
                    bindingContext.ModelState.Remove(key);
                }
            }
        }    
    }

Any better solutions?