How do I access other attribute values from inside a custom ValidationAttribute in an asp.net mvc 2 application?

Answer 1

Wait for MVC 3. I'm being serious.

The options for class wide custom validation attributes are pretty poor right now without any really good extensibility points. Your pretty much stuck creating a custom ModelBinder, which can then add values to ModelState, to do anything complex with validation attributes.

Use the attributes just like you would and then detect what Types are requested from the binder, Reflect to find the attributes, and then validated/add to model state as necessary.

MVC 3 fixes this problem but until then your stuck creating your own binder.

Answer 2

You could use a class attribute to achieve this:

[AttributeUsage(AttributeTargets.Class)]
public class RatingValidation : ValidationAttribute
{
    public override bool IsValid(object value)
    {
        var model = (MyModel)value;
        // TODO: here you have the model so work with the 
        // Rating and Heading properties to perform your 
        // validation logic

        return true;
    }
}

And you model:

[RatingValidation]
public class MyModel
{
    public String Heading { get; set; }
    public int Rating { get; set; }
}

Or even better: use FluentValidation which integrates nicely with ASP.NET MVC.

Answer 3

Better phrasing of this question altogether:

How can I get a distinct error message to show for the validation summary for a given form field with a separate & distinct message showing beside the field for which the validation applies?

Example:

Field: Rating
Summary Message: "Rating for Technical Knowledge is required and must be between 1 and 5." Field Validation Message: "*"

Hopefully that illustrates what I'm trying to do. From the research the answer seems to be this isn't possible with the current data annotations framework without rolling at least one html helper on my own: either create my own ValidationSummary or my own ValidationMessage helper...

Answer 4

I wanted to elaborate on jfar's suggestion. I am not agreeing that I need to wait till MVC 3; however, his point for creating a custom model binder is actually THE ONLY way to do what I am trying to do. I've tried both field/property level attributes and class level attributes. Neither of these are sufficient for the sort of messages I need.

For example, recall that my situation is that I have a template control that is strongly typed against a model with properties Index, Heading, & Rating. So a page of these controls displayed would be something like:

Heading: Technical Knowledge Rating: [text box]

Heading: Leadership Rating: [text box]

Heading: Work Ethic Rating: [text box]

...etc, etc.

For my validation messages, I wanted something very customized. It turns out my needs were too specific for the out of the box validating in MVC 2. I needed error messages for the Rating field to reference the Heading that Rating is associated with. So the validation would be something like:

The Rating for Work Ethic is required and must be a number between 1 and 5. The Rating value of 'asdf' for Work Ethic is not valid.

The issue with field level attributes is that they didn't have access to the Heading value. Furthermore, each object actually contains an IsEditable field and should that field's value be false, I bypass validations altogether.

The issue with class level attributes is two fold. For one, I can't control the key used in the ModelStateCollection. The default is the class name and index of the object on the page. This yields a result akin to: PersonRating[0], PersonRating[1]. The problem with this is that it means you can only have 1 error message at a class level validation. If you have 2 class level attributes, they both get put into the ModelStateCollection with the same key. I'm not really sure how that works as I wouldn't think a dictionary would let you do that. Perhaps it fails silently or the second message just overwrites the first. In addition to this, I still need the fields themselves to have their css changes to denote an error. With class level validations, I could not figure out how to do this because the key does not reference a field...so I have no idea which message goes with what field unless I do hard string checks, which seems a really bad solution.

The blog post I referenced earlier does provide a solution, but it requires far too much code and effort per attribute and just seemed like overkill.

In addition to all of this, MVC data annotations has basic validations hard wired into it to sort of save you from yourself. This includes invoking a Required validator if a type on your model is non-nullable. The internal MVC validations also do data checking to ensure you're not trying to submit a string value to a int type. In both of these cases, I didn't not see a viable way to correct the validation message without doing hard checks against the validation text.

So, in the end, I wrote my own model binder for this object. While this is not the recommended way, I take comfort in that this really is the only viable solution in my case due to the internal validations hardwired in the MVC framework. My model binder looked something like below:

        public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
        {
            Object o = base.BindModel(controllerContext, bindingContext);
            string ratingKey = bindingContext.ModelName + ".Rating";            
            PersonRating pr = (PersonRating)o;
            ValueProviderResult ratingVpr = controllerContext.
                                        Controller.
                                            ValueProvider.
                                                GetValue(ratingKey);
            String ratingVal = ratingVpr.AttemptedValue;
            String ratingErrorMessage = getRatingModelErrorMessage(
                                            ratingKey,
                                            ratingVal,
                                            pr);

            if (!String.IsNullOrEmpty(ratingErrorMessage))
            {
                bindingContext.ModelState[ratingKey].Errors.Clear();
                bindingContext.ModelState.AddModelError(ratingKey, ratingErrorMessage);
            }

            return o;

                         }

The getRatingModelErrorMessage method is a custom method that performs validations on the Rating field of the PersonRating object and returns a string that represents the error message. If the string is null, no error was returned by the getRatingModelErrorMessage method.

I'll be the first to admit this isn't great code. There's always room to improve. Still it gets the job done. It's also worth noting that in situations where a value such as a text value is submitted for a non compatible type on the model, such as int, it will not be bound to the model. I am getting that value from the FormCollection via it's key.

Let me know if anyone has any suggestions on other ways this could be done or comments on the code in general. I'm always open for constructive criticism.