Fox.ValidationKit 1.0.0

Fox.ValidationKit

Lightweight .NET validation library with fluent API and zero dependencies

Fox.ValidationKit is a lightweight, expressive validation library for .NET with a fluent API. It provides strongly-typed validation rules with support for synchronous and asynchronous validation, custom rules, and optional ResultKit integration.

Installation

dotnet add package Fox.ValidationKit

NuGet Package Manager:

Install-Package Fox.ValidationKit

PackageReference:

<PackageReference Include="Fox.ValidationKit" Version="1.0.0" />

Quick Start

1. Define Your Model

public sealed class User
{
    public string? FirstName { get; set; }
    public string? LastName { get; set; }
    public string? Email { get; set; }
    public int Age { get; set; }
}

2. Create a Validator

public sealed class UserValidator : Validator<User>
{
    public UserValidator()
    {
        RuleFor(x => x.FirstName)
            .NotEmpty("First name is required")
            .MinLength(2)
            .MaxLength(50);

        RuleFor(x => x.LastName)
            .NotEmpty("Last name is required")
            .MinLength(2)
            .MaxLength(50);

        RuleFor(x => x.Email)
            .NotEmpty("Email is required")
            .EmailAddress("Invalid email format");

        RuleFor(x => x.Age)
            .GreaterThan(0, "Age must be positive")
            .LessThan(150, "Age must be realistic")
            .Custom((user, age) => age >= 18, "User must be at least 18 years old");
    }
}

3. Validate Your Data

var validator = new UserValidator();
var user = new User
{
    FirstName = "John",
    LastName = "Doe",
    Email = "john.doe@example.com",
    Age = 25
};

var result = validator.Validate(user);

if (result.IsValid)
{
    Console.WriteLine("User is valid!");
}
else
{
    foreach (var error in result.Errors)
    {
        Console.WriteLine($"{error.PropertyName}: {error.Message}");
    }
}

Features

• Fluent API - Intuitive, type-safe validation rule configuration
• Zero Dependencies - No external dependencies, only .NET BCL
• Strongly Typed - Expression-based property selection with IntelliSense
• 15+ Built-in Rules - NotNull, NotEmpty, GreaterThan, LessThan, Between, Matches, EmailAddress, Url, CreditCard, etc.
• Custom Rules - Synchronous and asynchronous custom validation logic
• Error Codes - FVK### error codes for localization and structured error handling
• Async Support - ValidateAsync for asynchronous validation scenarios
• Collection Validation - NotEmpty, MinCount, MaxCount, RuleForEach
• Conditional Validation - When and Unless for conditional rules
• Cascade Modes - Continue or Stop validation on first failure
• Nested Validation - SetValidator for complex objects
• Localization Ready - IValidationMessageProvider for custom messages

Documentation

Full documentation available at: https://github.com/akikari/Fox.ValidationKit

License

Fox.ValidationKit is licensed under the MIT License.

Copyright (c) 2026 Karoly Akacz

Author

Karoly Akacz

• GitHub: @akikari
• Project: https://github.com/akikari/Fox.ValidationKit