Customizing ASP.NET MVC’s Required Property Validation Messages

6 commentsWritten on April 3rd, 2011 by
Categories: ASP.NET MVC

I recently got convention-based localization of display labels working with ASP.NET MVC, and this week, i wanted to get something similar working for required field validation messages. ASP.NET MVC3 shows a default validation message when a required field is not filled in, unless you specify a resource provider and the name of the resource key when you put the Required attribute on a property. Just like with the display value of labels, i wanted a convention based approach for this. I wanted ASP.NET MVC to look for a resource key with the NameOfModelClass_NameOfProperty_required convention.

After some googling and browsing the MVC3 source code, i couldn't really find the hook i needed to make this happen, so i let it rest for a few days and got back to it later on. I had more luck the second time around and found the hook i needed. ASP.NET MVC uses the RequiredAttributeAdapter class to retrieve a ModelClientValidationRequiredRule which by default is initialized with the default error message. The trick was just to inherit from this class and return a ModelClientValidationRequiredRule with your own message, and then register that class with the DataAnnotationsModelValidatorProvider. This is the new subclass of the RequiredAttributeAdapter class:

    public class ConventionsBasedRequiredAttributeAdapter : RequiredAttributeAdapter
    {
        public ConventionsBasedRequiredAttributeAdapter(ModelMetadata metadata, ControllerContext context, RequiredAttribute attribute) 
            : base(metadata, context, attribute) {}

        public override IEnumerable<ModelClientValidationRule> GetClientValidationRules()
        {
            string errorMessage;

            var className = Metadata.ContainerType.Name;
            var propertyName = Metadata.PropertyName;

            var specificKey = string.Format("{0}_{1}_required", className, propertyName);
            // TODO: make the ResourceManager configurable
            errorMessage = Resources.ResourceManager.GetObject(specificKey) as string;

            if (string.IsNullOrEmpty(errorMessage))
            {
                var genericMessageWithPlaceHolder = (string)Resources.ResourceManager.GetObject("Generic_required_field_message");

                if (!string.IsNullOrEmpty(genericMessageWithPlaceHolder))
                {
                    errorMessage = string.Format(genericMessageWithPlaceHolder, Metadata.DisplayName);
                }
            }

            if (string.IsNullOrEmpty(errorMessage))
            {
                errorMessage = ErrorMessage; // fallback to what ASP.NET MVC would normally display
            }

            return new[] { new ModelClientValidationRequiredRule(errorMessage) };
        }
    }

And this is how you tell ASP.NET MVC to use it:

            DataAnnotationsModelValidatorProvider.RegisterAdapterFactory(typeof(RequiredAttribute), 
                (metadata, controllerContext, attribute) => new ConventionsBasedRequiredAttributeAdapter(metadata, 
                    controllerContext, (RequiredAttribute)attribute));

And that's it... with this approach you have full control over how the message is formatted.

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This entry was posted on Sunday, April 3rd, 2011 at 19:45 and is filed under ASP.NET MVC. You can subscribe to any comments to this entry via RSS 2.0. You can leave a response, or trackback from your own site.

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