AssertValue is a validation attribute (i.e. a data annotation) that can be applied to a property of a view model class. It causes the property to be invalid unless it has the value specified in the attribute's constructor. The attribute implements IClientValidatable which ties it up with client-side JavaScript in order to replicate the validatio…

assertvalue.js

// The following code may not work if executed after the page has loaded and the validator has initialised.
// That may require further investigation, but it certainly works if executed at the top of the page.

// Create the assertvalue validator method
jQuery.validator.addMethod("assertvalue", function (value, element, param) {
    if (value.toLowerCase && param.toLowerCase)
        return value.toLowerCase() == param.toLowerCase();

    return value == param;
});

// Add the assertvalue adapter, tying in with the method just added and using the asserted parameter.
// These names must match the ones used by the server-side AssertValueAttribute.
jQuery.validator.unobtrusive.adapters.addSingleVal("assertvalue", "asserted");

AssertValueAttribute.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using System.ComponentModel;
using System.ComponentModel.DataAnnotations;

namespace Validators
{
    [AttributeUsage(AttributeTargets.Property, AllowMultiple = false)]
    public class AssertValueAttribute : ValidationAttribute, IClientValidatable
    {
        public object AssertedValue { get; protected set; }

        public AssertValueAttribute(object value)
        {
            AssertedValue = value;
        }

        protected override ValidationResult IsValid(object value, ValidationContext validationContext)
        {
            bool match = false;

            try
            {
                match = AssertedValue.Equals(value);
            }
            catch (Exception e) { }

            if (match)
                return ValidationResult.Success;
            else
                return new ValidationResult(FormatErrorMessage(validationContext.DisplayName));
        }

        public IEnumerable<ModelClientValidationRule> GetClientValidationRules(ModelMetadata metadata, ControllerContext context)
        {
            var rule = new ModelClientValidationRule
            {
                ErrorMessage = this.ErrorMessage,
                ValidationType = "assertvalue"
            };

            rule.ValidationParameters.Add("asserted", AssertedValue);
            yield return rule;
        }
    }
}

SampleModel.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.ComponentModel;
using System.ComponentModel.DataAnnotations;
using System.Web.Mvc;

namespace ViewModels
{
    public class SampleModel
    {
        [AssertValue(true, ErrorMessage = "This property should have the value 'true'")]
        public bool MustBeTrue { get; set; }

        [AssertValue(false, ErrorMessage = "This property should have the value 'false'")]
        public bool MustBeFalse { get; set; }
    }
}