After some further debugging work I believe I understand why in my particular case the IDataErrorInfo.Item isn t being called. The following code is used in the ASP.NET MVC 2 Beta to validate an IDataErrorInfo property:

internal sealed class DataErrorInfoPropertyModelValidator : ModelValidator
{
    public DataErrorInfoPropertyModelValidator(ModelMetadata metadata, ControllerContext controllerContext)
        : base(metadata, controllerContext)
    {
    }

    public override IEnumerable<ModelValidationResult> Validate(object container)
    {
        if (Metadata.Model != null)
        {
            var castContainer = container as IDataErrorInfo;
            if (castContainer != null)
            {
                string errorMessage = castContainer[Metadata.PropertyName];
                if (!String.IsNullOrEmpty(errorMessage))
                {
                    return new[] { new ModelValidationResult { Message = errorMessage } };
                }
            }
        }
        return Enumerable.Empty<ModelValidationResult>();
    }
}

My model contains a property that is System.Nullable<int> and when the model binder binds an empty string from the HTML post, the Metadata.Model is equal to null, thus the validation doesn t run.

This is fundamentally different from ASP.NET MVC 1.0, where this scenario fires the validator all the way through to a call to IDataErrorInfo.Item.

Am I just not using something the way it was intended?

DefaultModelBinder in MVC 1.0:

protected virtual void OnPropertyValidated(ControllerContext controllerContext, ModelBindingContext bindingContext, PropertyDescriptor propertyDescriptor, object value)
{
    IDataErrorInfo model = bindingContext.Model as IDataErrorInfo;
    if (model != null)
    {
        string str = model[propertyDescriptor.Name];
        if (!string.IsNullOrEmpty(str))
        {
            string key = CreateSubPropertyName(bindingContext.ModelName, propertyDescriptor.Name);
            bindingContext.ModelState.AddModelError(key, str);
        }
    }
}

DefaultModelBinder in MVC 2.0 beta:

protected virtual void OnPropertyValidated(ControllerContext controllerContext, ModelBindingContext bindingContext, PropertyDescriptor propertyDescriptor, object value)
{
    ModelMetadata metadata = bindingContext.PropertyMetadata[propertyDescriptor.Name];
    metadata.Model = value;
    string prefix = CreateSubPropertyName(bindingContext.ModelName, metadata.PropertyName);
    foreach (ModelValidator validator in metadata.GetValidators(controllerContext))
    {
        foreach (ModelValidationResult result in validator.Validate(bindingContext.Model))
        {
            bindingContext.ModelState.AddModelError(CreateSubPropertyName(prefix, result.MemberName), result.Message);
        }
    }
    if ((bindingContext.ModelState.IsValidField(prefix) && (value == null)) && !TypeHelpers.TypeAllowsNullValue(propertyDescriptor.PropertyType))
    {
        bindingContext.ModelState.AddModelError(prefix, GetValueRequiredResource(controllerContext));
    }
}

It doesn t use IDataErrorInfo this[string columnName] property... Seems like a bug, because DefaultModelBinder still uses Error property. It is inconsistency at least.

EDIT

I used reflector and noticed that DataErrorInfoPropertyModelValidator doesn t seem to be used, so I created my own class:

public class DataErrorInfoModelPropertyValidatorProvider : ModelValidatorProvider
{
    // Methods
    public override IEnumerable<ModelValidator> GetValidators(ModelMetadata metadata, ControllerContext context)
    {
        if (metadata == null)
        {
            throw new ArgumentNullException("metadata");
        }
        if (context == null)
        {
            throw new ArgumentNullException("context");
        }

        var validators = new List<ModelValidator>();
        validators.Add(new DataErrorInfoPropertyModelValidator(metadata, context));
        return validators;
    }

    internal sealed class DataErrorInfoPropertyModelValidator : ModelValidator
    {
        // Methods
        public DataErrorInfoPropertyModelValidator(ModelMetadata metadata, ControllerContext controllerContext)
            : base(metadata, controllerContext)
        {
        }

        public override IEnumerable<ModelValidationResult> Validate(object container)
        {
            if (container != null)
            {
                IDataErrorInfo info = container as IDataErrorInfo;
                if (info != null)
                {
                    string str = info[Metadata.PropertyName];
                    if (!string.IsNullOrEmpty(str))
                    {
                        ModelValidationResult[] resultArray = new ModelValidationResult[1];
                        ModelValidationResult result = new ModelValidationResult();
                        result.Message = str;
                        resultArray[0] = result;
                        return resultArray;
                    }
                }
            }
            return Enumerable.Empty<ModelValidationResult>();
        }
    }
}

Then I used:

ModelValidatorProviders.Providers.Add(new DataErrorInfoModelPropertyValidatorProvider());

And it works:) This is just temporary solution. Will have to be corrected in final MVC 2.

EDIT

I also changed if (base.Metadata.Model != null) to if (container != null) in Validate() method of DataErrorInfoPropertyModelValidator.