Basic Rules

This document covers all synchronous rules available in IRuleBuilder<T, TProperty>. Async rules (MustAsync, DependentRuleAsync, Custom, Transform, SetValidator) are described in Advanced Rules.

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Nullability and Emptiness

NotNull()

Verifies that the value is not null.

RuleFor(x => x.Category).NotNull();
RuleFor(x => x.Tags).NotNull();

Null()

Verifies that the value is null. Useful for validating that certain fields must NOT be sent in specific requests.

RuleFor(x => x.UserId).Null()
    .WithMessage("Do not send the user ID in the body; it is taken from the token.");

NotEmpty()

Verifies that the value is not null, not an empty string (""), and not only whitespace. For collections, also verifies they are not empty.

RuleFor(x => x.Title).NotEmpty();
RuleFor(x => x.Content).NotEmpty();

Empty()

Verifies that the value is null or an empty string.

RuleFor(x => x.InternalNotes)
    .Empty()
        .WithMessage("Internal notes must not be sent from the public API.")
    .When(x => x.Source == RequestSource.PublicApi);

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Equality

EqualTo(TProperty value)

RuleFor(x => x.AcceptTerms)
    .EqualTo(true)
        .WithMessage("You must accept the terms and conditions.");

NotEqual(TProperty value)

RuleFor(x => x.NewPassword)
    .NotEqual("password")
        .WithMessage("Do not use 'password' as your password.");

EqualToProperty(Expression<Func<T, TProperty>> otherProp)

Verifies that the value equals the value of another property. Ideal for password or email confirmation.

RuleFor(x => x.ConfirmPassword)
    .NotEmpty()
    .EqualToProperty(x => x.NewPassword)
        .WithMessage("The passwords do not match.");

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String Length

MinimumLength(int n)

RuleFor(x => x.Username).MinimumLength(3);
RuleFor(x => x.Password).MinimumLength(8);

MaximumLength(int n)

RuleFor(x => x.Name).MaximumLength(200);
RuleFor(x => x.Email).MaximumLength(320); // RFC 5321 limit

LengthBetween(int min, int max)

RuleFor(x => x.PhoneNumber)
    .LengthBetween(7, 15)
        .WithMessage("The phone number must have between 7 and 15 digits.");

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Numeric Range

These rules work with IComparable — they work with int, decimal, double, DateTime, long, etc.

GreaterThan / GreaterThanOrEqualTo

RuleFor(x => x.Age).GreaterThan(0);
RuleFor(x => x.Stock).GreaterThanOrEqualTo(0);

LessThan / LessThanOrEqualTo

RuleFor(x => x.Age).LessThan(150);
RuleFor(x => x.Percentage).LessThanOrEqualTo(100m);

Between / ExclusiveBetween

RuleFor(x => x.Month).Between(1, 12)
    .WithMessage("The month must be between 1 and 12.");

RuleFor(x => x.Probability)
    .ExclusiveBetween(0m, 1m)
        .WithMessage("The probability must be between 0 and 1 (extremes excluded).");

Positive()

Verifies that the number is strictly greater than 0. Zero is not positive.

RuleFor(x => x.Price).Positive();

NonNegative()

Verifies that the number is greater than or equal to 0 (zero is allowed).

RuleFor(x => x.Balance).NonNegative()
    .WithMessage("The balance cannot be negative.");

Percentage()

Verifies that the numeric value is between 0 and 100 inclusive.

RuleFor(x => x.DiscountRate)
    .Percentage()
        .WithMessage("The discount rate must be between 0 and 100.");

Precision(int totalDigits, int decimalPlaces)

Verifies that the decimal value fits within a DECIMAL(totalDigits, decimalPlaces) column.

RuleFor(x => x.Price)
    .Precision(10, 2)
        .WithMessage("The price cannot exceed 10 significant digits with 2 decimal places.");

Negative() / NotZero() / Odd() / Even() / MultipleOf() / MaxDecimalPlaces()

RuleFor(x => x.ScaleFactor).NotZero();
RuleFor(x => x.BatchSize).Even();
RuleFor(x => x.PageSize).MultipleOf(10);
RuleFor(x => x.Price).MaxDecimalPlaces(2);

MultipleOfProperty(Expression<Func<T, TProperty>> otherExpression)

Verifies that the value is a multiple of another property on the same object.

RuleFor(x => x.Quantity)
    .MultipleOfProperty(x => x.MinLotSize)
        .WithMessage("The order quantity must be a multiple of the minimum lot size.");

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Strings

Matches(string pattern)

RuleFor(x => x.PostalCode)
    .Matches(@"^\d{5}(-\d{4})?$")
        .WithMessage("The postal code must be a ZIP (XXXXX or XXXXX-XXXX).");

MustContain / NotContains / StartsWith / EndsWith

RuleFor(x => x.Username)
    .NotContains("admin", StringComparison.OrdinalIgnoreCase)
        .WithMessage("The username cannot contain 'admin'.");

RuleFor(x => x.Email)
    .EndsWith("@company.com", StringComparison.OrdinalIgnoreCase)
        .WithMessage("Only corporate emails are accepted.");

IsAlpha() / IsAlphanumeric() / IsNumeric()

RuleFor(x => x.FirstName).IsAlpha();
RuleFor(x => x.Username).IsAlphanumeric();
RuleFor(x => x.PinCode).IsNumeric().LengthBetween(4, 6);

Lowercase() / Uppercase() / NoWhitespace()

RuleFor(x => x.Slug).Lowercase();
RuleFor(x => x.CountryCode).Uppercase().LengthBetween(2, 3);
RuleFor(x => x.ApiKey).NoWhitespace();

MinWords(int n) / MaxWords(int n)

RuleFor(x => x.FullName).MinWords(2)
    .WithMessage("Please enter your full name (first and last name).");

Slug()

Verifies that the string is a valid URL slug: lowercase letters, digits and hyphens only.

RuleFor(x => x.UrlSlug)
    .Slug()
        .WithMessage("The slug can only contain lowercase letters, numbers and hyphens.");

IsValidJson() / IsValidBase64()

RuleFor(x => x.Metadata)
    .IsValidJson()
        .WithMessage("The metadata field must contain valid JSON.");

RuleFor(x => x.FileContent)
    .IsValidBase64()
        .WithMessage("The file content must be encoded in Base64.");

NoHtmlTags() / NoSqlInjectionPatterns()

RuleFor(x => x.Comment)
    .NoHtmlTags()
        .WithMessage("The comment cannot contain HTML tags.");

RuleFor(x => x.SearchQuery)
    .NoSqlInjectionPatterns()
        .WithMessage("The search query contains forbidden characters.");

Iban()

Verifies a valid IBAN using the mod-97 checksum algorithm.

RuleFor(x => x.BankAccount)
    .NotEmpty()
    .Iban()
        .WithMessage("The bank account number is not a valid IBAN.");

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Format and Data Validation

Email() / Url() / PhoneNumber()

RuleFor(x => x.Email).NotEmpty().Email();
RuleFor(x => x.Website).Url();
RuleFor(x => x.PhoneNumber).PhoneNumber()
    .WithMessage("The phone number must be in E.164 format (e.g. +12025551234).");

IPv4() / IPv6() / MacAddress()

RuleFor(x => x.ServerIp).IPv4();
RuleFor(x => x.ServerAddress).IPv6();
RuleFor(x => x.DeviceMacAddress).MacAddress();

CreditCard()

Verifies using the Luhn algorithm.

RuleFor(x => x.CardNumber)
    .NotEmpty()
    .CreditCard()
        .WithMessage("The card number is not valid.");

Guid() / NotEmptyGuid()

RuleFor(x => x.ExternalId).Guid();
RuleFor(x => x.UserId).NotEmptyGuid();

IsEnum<TEnum>()

RuleFor(x => x.Status)
    .IsEnum<OrderStatus>()
        .WithMessage("The status must be one of: Pending, Processing, Shipped...");

CountryCode() / CurrencyCode()

RuleFor(x => x.CountryCode).CountryCode()
    .WithMessage("The country code must be a valid ISO 3166-1 alpha-2 code (e.g. US, GB, ES).");
RuleFor(x => x.Currency).CurrencyCode()
    .WithMessage("The currency must be a valid ISO 4217 code (e.g. USD, EUR, GBP).");

Latitude() / Longitude()

RuleFor(x => x.Latitude).Latitude();
RuleFor(x => x.Longitude).Longitude();

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Password and Security

HasUppercase() / HasLowercase() / HasDigit() / HasSpecialChar()

RuleFor(x => x.Password)
    .NotEmpty()
    .MinimumLength(8)
    .HasUppercase().WithMessage("Must contain at least one uppercase letter.")
    .HasLowercase().WithMessage("Must contain at least one lowercase letter.")
    .HasDigit().WithMessage("Must contain at least one number.")
    .HasSpecialChar().WithMessage("Must contain at least one special character.");

PasswordPolicy(...)

A single all-in-one rule combining length, uppercase, lowercase, digit and special-character checks.

// Default policy: 8+ chars, upper, lower, digit, special char
RuleFor(x => x.Password).PasswordPolicy();

// Stricter enterprise policy
RuleFor(x => x.Password)
    .PasswordPolicy(minLength: 12, requireSpecialChar: true)
        .WithMessage("The password does not meet the security policy.");

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Dates

FutureDate() / PastDate() / Today()

RuleFor(x => x.EventDate).FutureDate();
RuleFor(x => x.BirthDate).PastDate();
RuleFor(x => x.ReportDate).Today();

MinAge(int years) / MaxAge(int years)

RuleFor(x => x.BirthDate)
    .PastDate()
    .MinAge(18)
        .WithMessage("You must be at least 18 years old to register.");

DateBetween(DateTime from, DateTime to)

RuleFor(x => x.AppointmentDate)
    .DateBetween(DateTime.Today, DateTime.Today.AddMonths(3))
        .WithMessage("Appointments can be booked up to 3 months in advance.");

NotExpired() / WithinNext(TimeSpan) / WithinLast(TimeSpan)

RuleFor(x => x.PassportExpiryDate).NotExpired();
RuleFor(x => x.AppointmentDate).WithinNext(TimeSpan.FromDays(30));
RuleFor(x => x.TestDate).WithinLast(TimeSpan.FromDays(180));

IsWeekday() / IsWeekend()

RuleFor(x => x.AppointmentDate).IsWeekday()
    .WithMessage("Appointments can only be scheduled on weekdays (Monday–Friday).");

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Collections

NotEmptyCollection() / HasCount() / MinCount() / MaxCount()

RuleFor(x => x.OrderLines).NotEmptyCollection()
    .WithMessage("The order must have at least one product.");
RuleFor(x => x.Tags).MaxCount(5)
    .WithMessage("You cannot add more than 5 tags.");

Unique()

RuleFor(x => x.SelectedRoleIds).Unique()
    .WithMessage("The selected roles cannot be repeated.");

AllSatisfy() / AnySatisfy()

RuleFor(x => x.FileNames)
    .AllSatisfy(name => ((string)name).EndsWith(".pdf"))
        .WithMessage("All files must be PDF.");

In() / NotIn()

private static readonly string[] AllowedCurrencies = new[] { "EUR", "USD", "GBP", "JPY" };

RuleFor(x => x.Currency)
    .In(AllowedCurrencies)
        .WithMessage("The currency must be EUR, USD, GBP or JPY.");

RuleFor(x => x.Username)
    .NotIn(new[] { "admin", "root", "system" })
        .WithMessage("That username is reserved.");

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Summary of Available Rules

Category	Rules
Null/empty	NotNull, Null, NotEmpty, Empty
Equality	EqualTo, NotEqual, EqualToProperty
Length	MinimumLength, MaximumLength, LengthBetween
Numeric range	GreaterThan, GreaterThanOrEqualTo, LessThan, LessThanOrEqualTo, Between, ExclusiveBetween, Positive, NonNegative, Negative, NotZero, Odd, Even, MultipleOf, MultipleOfProperty, MaxDecimalPlaces, Percentage, Precision
String	Matches, MustContain, NotContains, StartsWith, EndsWith, IsAlpha, IsAlphanumeric, IsNumeric, Lowercase, Uppercase, NoWhitespace, MinWords, MaxWords, Slug, NoHtmlTags, NoSqlInjectionPatterns
Format	Email, Url, PhoneNumber, IPv4, IPv6, MacAddress, CreditCard, Guid, NotEmptyGuid, IsEnum<T>, CountryCode, CurrencyCode, Latitude, Longitude, IsValidJson, IsValidBase64, Iban
Password	HasUppercase, HasLowercase, HasDigit, HasSpecialChar, PasswordPolicy
Dates	FutureDate, PastDate, Today, MinAge, MaxAge, DateBetween, NotExpired, WithinNext, WithinLast, IsWeekday, IsWeekend
Collections	NotEmptyCollection, HasCount, MinCount, MaxCount, Unique, AllSatisfy, AnySatisfy, In, NotIn

Next Steps

• Advanced Rules — Must, MustAsync, Custom, Transform, SetValidator
• Modifiers — WithMessage, WithErrorCode, When/Unless and more