Why does ASP.NET MVC care about my read only properties during databinding?

Edit: Added bounty because I'm seeking an MVC3 solution (if one exists) other than this:

DataAnnotationsModelValidatorProvider.AddImplicitRequiredAttributeForValueTypes = false;

I have a read only property on my 'Address' model 'CityStateZip'.

It's just a convenient way to get city, state, zip from a US address. It throws an exception if the country is not USA (the caller is supposed to check first).

    public string CityStateZip
    {
        get
        {
            if (IsUSA == false)
            {
                throw new ApplicationException("CityStateZip not valid for international addresses!");
            }

            return (City + ", " + StateCd + " " + ZipOrPostal).Trim().Trim(new char[] {','});
        }
    }

This is part of my model so it gets bound. Prior to ASP.NET MVC2 RC2 this field never caused a problem during databinding. I never even really thought about it - after all it is only read only.

Now though with the January 2010 RC2 release it gives me an error during databinding - becasue the default model binder seems to want to check this value (even though it is read only).

It is the 'base.OnModelUpdated' line that causes this error to be triggered.

public class AddressModelBinder : DefaultModelBinder
{
    protected override void OnModelUpdated(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
        base.OnModelUpdated(controllerContext, bindingContext);

Last minutes changes to the modelbinder evidently caused this change in behavior - but I'm not quite sure yet what the repurcussions of it are - or whether or not this is a bug? I'm passing this on to the MVC team but curious if anyone else has any suggestions in the meantime how I can prevent this property from binding.

This article is well worth reading about the changes - but doesn't mention readonly properties at all (not that I would expect it to). The issue (if there is one) may be broader than this situation - I'm just not sure about any repurcussions - if any!

Input Validation vs. Model Validation in ASP.NET MVC

As requested by @haacked here's the stacktrace :

I get this by simply adding the following line to ANY model and making a post to the corresponding action method. In this instance I added it to my simplest possible model.

 public string Foo { get { throw new Exception("bar"); } }

[TargetInvocationException: Property accessor 'Foo' on object 'Rolling_Razor_MVC.Models.ContactUsModel' threw the following exception:'bar'] System.ComponentModel.ReflectPropertyDescriptor.GetValue(Object component) +390 System.Web.Mvc.<>c__DisplayClassb.<GetPropertyValueAccessor>b__a() +18 System.Web.Mvc.ModelMetadata.get_Model() +22 System.Web.Mvc.ModelMetadata.get_RealModelType() +29 System.Web.Mvc.<GetValidatorsImpl>d__0.MoveNext() +38 System.Linq.<SelectManyIterator>d__14`2.MoveNext() +273 System.Web.Mvc.<Validate>d__5.MoveNext() +644 System.Web.Mvc.DefaultModelBinder.OnModelUpdated(ControllerContext controllerContext, ModelBindingContext bindingContext) +92 System.Web.Mvc.DefaultModelBinder.BindComplexElementalModel(ControllerContext controllerContext, ModelBindingContext bindingContext, Object model) +60 System.Web.Mvc.DefaultModelBinder.BindComplexModel(ControllerContext controllerContext, ModelBindingContext bindingContext) +1048 System.Web.Mvc.DefaultModelBinder.BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext) +280 System.Web.Mvc.Controller.TryUpdateModel(TModel model, String prefix, String[] includeProperties, String[] excludeProperties, IValueProvider valueProvider) +449 System.Web.Mvc.Controller.TryUpdateModel(TModel model) +73

Solution

I believe I'm experiencing a similar issue. I've posted the details:

http://forums.asp.net/t/1523362.aspx

edit: Response from MVC team (from above URL):

We investigated this and have concluded that the validation system is behaving as expected. Since model validation involves attempting to run validation over all properties, and since non-nullable value type properties have an implicit [Required] attribute, we're validating this property and calling its getter in the process. We understand that this is a breaking change from V1 of the product, but it's necessary to make the new model validation system operate correctly.

You have a few options to work around this. Any one of these should work:

Change the Date property to a method instead of a property; this way it will be ignored by the MVC framework.
Change the property type to DateTime? instead of DateTime. This removes the implicit [Required] from this property.
Clear the static DataAnnotationsModelValidatorProvider.AddImplicitRequiredAttributeForValueTypes flag. This removes the implicit [Required] from all non-nullable value type properties application-wide. We're considering adding in V3 of the product an attribute which will signal to us "don't bind it, don't validate it, just pretend that this property doesn't exist."

Thanks again for the report!