using System;
using FluentValidation;
using FluentValidation.Validators;

namespace FluentValidationSample.Models
{
    public class LowerThanValidator : PropertyValidator
    {
        public string DependentProperty { get; set; }

        public LowerThanValidator(string dependentProperty) : base($"باید کمتر از {dependentProperty} باشد")
        {
            DependentProperty = dependentProperty;
        }

        protected override bool IsValid(PropertyValidatorContext context)
        {
            if (context.PropertyValue == null)
            {
                return false;
            }

            var typeInfo = context.Instance.GetType();
            var dependentPropertyValue =
                Convert.ToInt32(typeInfo.GetProperty(DependentProperty).GetValue(context.Instance, null));

            return int.Parse(context.PropertyValue.ToString()) < dependentPropertyValue;
        }
    }

    public static class CustomFluentValidationExtensions
    {
        public static IRuleBuilderOptions<T, int> LowerThan<T>(
            this IRuleBuilder<T, int> ruleBuilder, string dependentProperty)
        {
            return ruleBuilder.SetValidator(new LowerThanValidator(dependentProperty));
        }
    }
}

using FluentValidation;
using FluentValidation.AspNetCore;
using FluentValidation.Internal;
using FluentValidation.Resources;
using FluentValidation.Validators;
using Microsoft.AspNetCore.Mvc.ModelBinding.Validation;

namespace FluentValidationSample.Models
{
    public class LowerThanClientValidator : ClientValidatorBase
    {
        private LowerThanValidator LowerThanValidator
        {
            get { return (LowerThanValidator)Validator; }
        }

        public LowerThanClientValidator(PropertyRule rule, IPropertyValidator validator) :
            base(rule, validator)
        {
        }

        public override void AddValidation(ClientModelValidationContext context)
        {
            MergeAttribute(context.Attributes, "data-val", "true");
            MergeAttribute(context.Attributes, "data-val-LowerThan", GetErrorMessage(context));
            MergeAttribute(context.Attributes, "data-val-LowerThan-dependentproperty", LowerThanValidator.DependentProperty);
        }

        private string GetErrorMessage(ClientModelValidationContext context)
        {
            var formatter = ValidatorOptions.MessageFormatterFactory().AppendPropertyName(Rule.GetDisplayName());
            string messageTemplate;
            try
            {
                messageTemplate = Validator.Options.ErrorMessageSource.GetString(null);
            }
            catch (FluentValidationMessageFormatException)
            {
                messageTemplate = ValidatorOptions.LanguageManager.GetStringForValidator<NotEmptyValidator>();
            }
            return formatter.BuildMessage(messageTemplate);
        }
    }
}

<input dir="rtl" type="number"
 data-val="true"
 data-val-lowerthan="باید کمتر از Age باشد"
 data-val-lowerthan-dependentproperty="Age"
 data-val-required="'سابقه کار' must not be empty."
 id="Experience"
 name="Experience"
 value="">

    public class UserModel
    {
        [Display(Name = "نام کاربری")]
        public string Username { get; set; }

        [Display(Name = "سن")]
        public int Age { get; set; }

        [Display(Name = "سابقه کار")]
        public int Experience { get; set; }
    }

    public class UserValidator : AbstractValidator<UserModel>
    {
        public UserValidator()
        {
            RuleFor(x => x.Username).NotNull();
            RuleFor(x => x.Age).NotNull();
            RuleFor(x => x.Experience).LowerThan(nameof(UserModel.Age)).NotNull();
        }
    }

namespace FluentValidationSample.Web
{
    public class Startup
    {
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddControllersWithViews().AddFluentValidation(
                fv =>
                {
                    fv.RegisterValidatorsFromAssembly(Assembly.GetExecutingAssembly());
                    fv.RegisterValidatorsFromAssemblyContaining<RegisterModelValidator>();
                    fv.RunDefaultMvcValidationAfterFluentValidationExecutes = false;

                    fv.ConfigureClientsideValidation(clientSideValidation =>
                    {
                        clientSideValidation.Add(
                            validatorType: typeof(LowerThanValidator),
                            factory: (context, rule, validator) => new LowerThanClientValidator(rule, validator));
                    });
                }
            );
        }

<script src="~/lib/jquery/dist/jquery.min.js"></script>
<script src="~/lib/bootstrap/dist/js/bootstrap.bundle.min.js"></script>
<script src="~/lib/jquery-validation/dist/jquery.validate.js"></script>
<script src="~/lib/jquery-validation-unobtrusive/jquery.validate.unobtrusive.js"></script>
<script src="~/js/site.js" asp-append-version="true"></script>

$.validator.unobtrusive.adapters.add('LowerThan', ['dependentproperty'], function (options) {
    options.rules['LowerThan'] = {
        dependentproperty: options.params['dependentproperty']
    };
    options.messages['LowerThan'] = options.message;
});

$.validator.addMethod('LowerThan', function (value, element, parameters) {      
    var dependentProperty = '#' + parameters['dependentproperty'];
    var dependentControl = $(dependentProperty);
    if (dependentControl) {
        var targetvalue = dependentControl.val();
        if (parseInt(targetvalue) > parseInt(value)) {
            return true;
        }
        return false;
    }
    return true;
});

@using FluentValidationSample.Models
@model UserModel
@{
    ViewData["Title"] = "Home Page";
}
<div dir="rtl">
    <form asp-controller="Home"
          asp-action="RegisterUser"
          method="post">
        <fieldset class="form-group">
        <legend>ثبت نام</legend>
            <div class="form-group row">
                <label asp-for="Username" class="col-md-2 col-form-label text-md-left"></label>
                <div class="col-md-10">
                    <input dir="rtl" asp-for="Username" class="form-control" />
                    <span asp-validation-for="Username" class="text-danger"></span>
                </div>
            </div>
            <div class="form-group row">
                <label asp-for="Age" class="col-md-2 col-form-label text-md-left"></label>
                <div class="col-md-10">
                    <input dir="rtl" asp-for="Age" class="form-control" />
                    <span asp-validation-for="Age" class="text-danger"></span>
                </div>
            </div>
            <div class="form-group row">
                <label asp-for="Experience" class="col-md-2 col-form-label text-md-left"></label>
                <div class="col-md-10">
                    <input dir="rtl" asp-for="Experience" class="form-control" />
                    <span asp-validation-for="Experience" class="text-danger"></span>
                </div>
            </div>

            <div class="form-group row">
                <label class="col-md-2 col-form-label text-md-left"></label>
                <div class="col-md-10 text-md-right">
                    <button type="submit" class="btn btn-info col-md-2">ارسال</button>
                </div>
            </div>
        </fieldset>
    </form>
</div>