Tuesday, March 12, 2013

Custom Validation Attribute - Client Side Enabled!

In previous posts, I outlined how to make a custom validation attribute - including a more complex one that depends on another property. Both of those validators are working - but they are working on the server-side. So if you have an ASP.NET MVC web application, the validation will fire after it gets to the server side within your controller. Ideally, we want to do client-side validation as well - so that the form does not need to POST if the data is not valid. How do we do that?

We can make custom javascript, query the fields manually and check for conditions - but this code will be disconnected from the custom validator that we created. Doing custom javascript also means that it will be so specific that the likelihood of being reusable is very very small.

Also, as we have seen with the built-in validators that shipped with ASP.NET MVC, they seem to just work - no custom javascript needed per field or manual client-side validation. When a property or a field is decorated with the correct attribute and client-side validation is enabled in the web.config, then boom - it just works. How can we make our validator to behave in similar manner?

One way to do this is by implementing IClientValidatable in your custom validator. Implementing this interface means we must implement a method called "GetClientValidationRules". This method is what basically will become a connector for our validator to the client-side. The property decorated with our custom validation attribute will then outputting flags that will indicate that it should be evaluated during client-side validation. In this method as well we need to provide additional information to properly evaluate the validity of the property on the client-side - mostly a (javascript) method name to be called during validation and some parameters needed to evaluate property within that method. When we do this, this is the updated validator class will look like:
[AttributeUsage(AttributeTargets.Property | AttributeTargets.Field, AllowMultiple = false)]
sealed public class MagicNumberAttribute : ValidationAttribute, IClientValidatable {
   // constructor to accept the comparison property name
   public MagicNumberAttribute(string thirtySevenProperty) : base() {
      if (thirtySevenProperty == null) {
         throw new ArgumentNullException("thirtySevenProperty");
      ThirtySevenProperty= thirtySevenProperty;

   // property to store the comparison field name
   public string ThirtySevenProperty{ get; private set; }

   protected override ValidationResult IsValid(object value, ValidationContext validationContext) {
      // get property info of the "ThirtySevenProperty"
      PropertyInfo thirtySevenPropertyInfo = validationContext.ObjectType.GetProperty(ThirtySevenProperty);

      // check if that property exists / valid
      if (thirtySevenPropertyInfo == null) {
         return new ValidationResult(String.Format(CultureInfo.CurrentCulture, "UNKNOWN PROPERTY", ThirtySevenProperty));

      // get the value of the property
      object thirtySevenPropertyValue = thirtySevenPropertyInfo.GetValue(validationContext.ObjectInstance, null);

      // do comparison and return validation result with error if not equal
      if (!Equals(value, thirtySevenPropertyValue)) {
         return new ValidationResult(FormatErrorMessage(validationContext.DisplayName));

      // return null if everything is ok
      return null;

   public IEnumerable<ModelClientValidationRule$gt; GetClientValidationRules(ModelMetadata metadata, ControllerContext context) {
      yield return new ModelClientValidationEqualToRule(this.ErrorMessage, OtherProperty);        
In the code above, I am reusing an existing validation rule "ModelClientValidationEqualToRule" which will connect with "equalto" jQuery client-validator. This is a pre-existing rule, so at this point, we can just run this and it should work - no more code needed.

In the next post, I will go over on how to return our own custom ModelClientValidationRule and how to create a corresponding javascript client-side validator.

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