Question

I have the following rules

the 1st does work using unobtrusive, client side validation, the second does not

any ideas why?

RuleFor(x => x.StartDate)
    .LessThanOrEqualTo(x => x.EndDate.Value)
    .WithLocalizedMessage(() => CommonRes.Less_Than_Or_Equal_To, filters => CommonRes.Start_Date, filters => CommonRes.End_Date);

RuleFor(x => x.StartDate)
    .GreaterThanOrEqualTo(x => x.AbsoluteStartDate)
    .LessThanOrEqualTo(x => x.AbsoluteEndDate)
    .WithLocalizedMessage(() => CommonRes.Between, filters => CommonRes.Start_Date, filters => filters.AbsoluteStartDate, filters => filters.AbsoluteEndDate);

Answer 1

Neither of the LessThanOrEqualTo or GreaterThanOrEqualTo rules are supported by client side validation as explained in the documentation.

This means that if you want to have client side validation for them you will need to write a custom FluentValidationPropertyValidator and implement the GetClientValidationRules method which will allow you to register a custom adapter and implement the client side validation logic for it in javascript.

If you are interested on how this could be achieved just ping me and I will provide an example.

---

Update

As request, I will try to show an example of how one could implement custom client side validation for the LessThanOrEqualTo rule. It is only a particular case with non-nullable dates. Writing such custom client side validator for all the possible case is of course possible but it will require significantly more efforts.

So we start with a view model and a corresponding validator:

[Validator(typeof(MyViewModelValidator))]
public class MyViewModel
{
    [Display(Name = "Start date")]
    [DisplayFormat(DataFormatString = "{0:yyyy-MM-dd}", ApplyFormatInEditMode = true)]
    public DateTime StartDate { get; set; }

    public DateTime DateToCompareAgainst { get; set; }
}

public class MyViewModelValidator : AbstractValidator<MyViewModel>
{
    public MyViewModelValidator()
    {
        RuleFor(x => x.StartDate)
            .LessThanOrEqualTo(x => x.DateToCompareAgainst)
            .WithMessage("Invalid start date");
    }
}

Then a controller:

public class HomeController : Controller
{
    public ActionResult Index()
    {
        var model = new MyViewModel
        {
            StartDate = DateTime.Now.AddDays(2),
            DateToCompareAgainst = DateTime.Now
        };
        return View(model);
    }

    [HttpPost]
    public ActionResult Index(MyViewModel model)
    {
        return View(model);
    }
}

and a view:

@model MyViewModel
@using (Html.BeginForm())
{
    @Html.Hidden("DateToCompareAgainst", Model.DateToCompareAgainst.ToString("yyyy-MM-dd"))

    @Html.LabelFor(x => x.StartDate)
    @Html.EditorFor(x => x.StartDate)
    @Html.ValidationMessageFor(x => x.StartDate)
    <button type="submit">OK</button>
}

All this is standard stuff so far. It will work but without client validation.

The first step is to write the FluentValidationPropertyValidator:

public class LessThanOrEqualToFluentValidationPropertyValidator : FluentValidationPropertyValidator
{
    public LessThanOrEqualToFluentValidationPropertyValidator(ModelMetadata metadata, ControllerContext controllerContext, PropertyRule rule, IPropertyValidator validator)
        : base(metadata, controllerContext, rule, validator)
    {
    }

    public override IEnumerable<ModelClientValidationRule> GetClientValidationRules()
    {
        if (!this.ShouldGenerateClientSideRules())
        {
            yield break;
        }

        var validator = Validator as LessThanOrEqualValidator;

        var errorMessage = new MessageFormatter()
            .AppendPropertyName(this.Rule.GetDisplayName())
            .BuildMessage(validator.ErrorMessageSource.GetString());

        var rule = new ModelClientValidationRule
        {
            ErrorMessage = errorMessage,
            ValidationType = "lessthanorequaldate"
        };
        rule.ValidationParameters["other"] = CompareAttribute.FormatPropertyForClientValidation(validator.MemberToCompare.Name);
        yield return rule;
    }
}

which will be registered in Application_Start when configuring our FluentValidation provider:

FluentValidationModelValidatorProvider.Configure(x =>
{
    x.Add(typeof(LessThanOrEqualValidator), (metadata, context, rule, validator) => new LessThanOrEqualToFluentValidationPropertyValidator(metadata, context, rule, validator));
});

And the last bit is the custom adapter on the client. So we add of course the 2 scripts to our page in order to enable unobtrusive client side validation:

<script src="@Url.Content("~/Scripts/jquery.validate.js")" type="text/javascript"></script>
<script src="@Url.Content("~/Scripts/jquery.validate.unobtrusive.js")" type="text/javascript"></script>

and the custom adapter:

(function ($) {
    $.validator.unobtrusive.adapters.add('lessthanorequaldate', ['other'], function (options) {
        var getModelPrefix = function (fieldName) {
            return fieldName.substr(0, fieldName.lastIndexOf(".") + 1);
        };

        var appendModelPrefix = function (value, prefix) {
            if (value.indexOf("*.") === 0) {
                value = value.replace("*.", prefix);
            }
            return value;
        }

        var prefix = getModelPrefix(options.element.name),
            other = options.params.other,
            fullOtherName = appendModelPrefix(other, prefix),
            element = $(options.form).find(":input[name=" + fullOtherName + "]")[0];

        options.rules['lessthanorequaldate'] = element;
        if (options.message != null) {
            options.messages['lessthanorequaldate'] = options.message;
        }
    });

    $.validator.addMethod('lessthanorequaldate', function (value, element, params) {
        var parseDate = function (date) {
            var m = date.match(/^(\d{4})-(\d{1,2})-(\d{1,2})$/);
            return m ? new Date(parseInt(m[1]), parseInt(m[2]) - 1, parseInt(m[3])) : null;
        };

        var date = parseDate(value);
        var dateToCompareAgainst = parseDate($(params).val());

        if (isNaN(date.getTime()) || isNaN(dateToCompareAgainst.getTime())) {
            return false;
        }

        return date <= dateToCompareAgainst;
    });

})(jQuery);