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

Question

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

DataAnnotationsModelValidatorProvider.AddImplicitRequiredAttributeForValueTypes = false;

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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

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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

Answer 1

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

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

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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!

Answer 2

Still having the same issue with MVC3.

I think the best way is to just to this in global.asax (from SevenCentral's answer):

 DataAnnotationsModelValidatorProvider.AddImplicitRequiredAttributeForValueTypes = false;

This will disable for all of them

Answer 3

This looks for all the world like a bug to me. I fully can't understand why the ModelBinder needs to check my read only properties (there may be some technicalities, but I definitely don't understand, and am not willing to spend the time trying to).

I added the following Model Meta Data Provider in to my solution to get round the problem

protected override CachedDataAnnotationsModelMetadata CreateMetadataPrototype(IEnumerable<Attribute> attributes, Type containerType, Type modelType, string propertyName)
{
    var metadata = base.CreateMetadataPrototype(attributes, containerType, modelType, propertyName);

    if (metadata.IsReadOnly)
    {
        metadata.IsRequired = false;
    }

    return metadata;
}

protected override CachedDataAnnotationsModelMetadata CreateMetadataFromPrototype(CachedDataAnnotationsModelMetadata prototype, Func<object> modelAccessor)
{
    var metadata = base.CreateMetadataFromPrototype(prototype, modelAccessor);

    if (prototype.IsReadOnly)
    {
        metadata.IsRequired = false;
    }

    return metadata;
}

You will also need to add the following to Global.asax.cs

protected void Application_Start()
{
    ModelMetadataProviders.Current = new RESModelMetadataProvider();
    ModelBinders.Binders.Add(typeof(SmartDate), new SmartDateModelBinder());

    ...
}

Answer 4

Of course I guess I could convert CityStateZip to GetCityStateZip() but then I cant bind it in something like silverlight quite as easily. This might work for a temporary fix for anyone else experiencing this issue.

Answer 5

I HAVE THE EXACT SAME PROBLEM!!

For more information about my issue, you may visit ASP.NET MVC 2.0 Unused Model Property being called when posting a product to the server?

does this mean we need to program our properties with the assumption that they'll be called unexpectedly (before properties which it depends on are set up/initialized etc)... if so, this represents a change in our programming practices and i would like to know how to proceed.

meanwhile, i just have a simple 'if' check that rids the problem.

Answer 6

I was having a similar problem, a field which I did not expect to be validated was receiving an error when the form posted back to the controller. After some googling, I came across http://codeblog.shawson.co.uk/mvc-strongly-typed-view-returns-a-null-model-on-post-back/ where it was pointed out that name conflicts could cause problems.

Though I did not think my post-back variable class had conflicting property names, renaming the property receiving the error solved my problem.

Answer 7

The same problem still exists in MVC 5.2.3, and it isn't even the validation code causing the problem. The DefaultModelBinder calls the getters outside of its validation code even if they are read-only properties and even if no request data is being passed to the controller matching those properties.

See a more detailed explanation and my full solution here: https://stackoverflow.com/a/54431404/10987278.

I'm not sure if that solution can be refactored to work with MVC 3, but hopefully it can help others still suffering from the same issue in the later version of MVC that stumbled across this question as I did.