FluentAnnotationsValidator 2.0.0-preview.2.3

FluentAnnotationsValidator

A fluent, type-safe validation engine for .NET that transforms [ValidationAttribute] annotations into runtime validation logic. Designed for ergonomic configuration, conditional logic, and culture-aware localization.

Version 2.0.0-preview.2: Introducing a Fluent API

This release marks a significant architectural shift by introducing a new, more expressive fluent API for defining validation rules. This new API provides greater flexibility for complex and conditional validation flows compared to the previous purely attribute-based approach.

New API and Behavior

• Rule(...): This method is now overloaded. It takes a property expression and an optional RuleDefinitionBehavior enum. By default, it replaces all previously registered rules for that property before adding the new one. The method returns a reference to the ValidationTypeConfigurator<T> instance, allowing you to configure the rule further.

• RuleFor(...): This new method takes a property expression and returns a reference to a new, property-specific builder (IValidationRuleBuilder<T, TProp>). It does not override existing rules for the property. If you want to override pre-registered rules, call the RemoveRulesFor<TProp>(Expression<Func<T, TProp>>) method. This new builder enables powerful, type-safe chaining of validation methods and conditional logic.

New Features

• Conditional Validation (When/Otherwise): The RuleFor(...) builder introduces a robust way to implement conditional logic.

  • When(condition, configureRules): This method allows you to group multiple validation rules that will only be evaluated if the provided condition is true.
  • Otherwise(configureRules): This method provides a corresponding set of rules that will be evaluated if the When condition is false, creating a clear if/else validation flow.

• Custom Validation (Must): The new Must(predicate) method is a key addition to the IValidationRuleBuilder<T, TProp>. It allows developers to define custom validation logic using a predicate (Func<TProp, bool>) that is executed on the property's value. This method is fully integrated into the fluent chaining, enabling complex validation rules that are not possible with standard attributes.

🌟 What's New in v2.0.0-preview1

FluentAnnotationsValidator v2.0.0-preview1 is a fresh rewrite. All legacy APIs from v1.x have been removed.

• Multi-attribute validation per property
• DSL-based configuration via ValidationConfigurator
• Conditional rules with .When(...), .Localized(...), .UseFallbackMessage(...)
• Convention-based registration from scanned assemblies
• Pluggable IValidationMessageResolver
• Scoped culture + resource binding per type
• Legacy support removed — clean slate architecture

To use the legacy version, pin to v1.2.2.

Quickstart

Basic Setup

Using AddFluentAnnotations():

using FluentAnnotationsValidator.Extensions;

services.AddFluentAnnotations();

Advanced Setup

Using either:

1. AddFluentAnnotationsValidators(...):

services.AddFluentAnnotationsValidators(typeof(LoginDto))
    .UseFluentAnnotations()
    .For<LoginDto>()
        .WithCulture(CultureInfo.GetCultureInfo("fr-FR"))
        .WithValidationResource<ValidationMessages>()
    .Build();

2. AddFluentAnnotations(...) with common behavior options configuration:

services.AddFluentAnnotations(
    configureBehavior: options =>
    {
        // common culture and resource type for all validation attributes
        options.CommonCulture = CultureInfo.GetCultureInfo("fr-FR");
        options.CommonResourceType = typeof(ValidationMessages);
    }
);

3. AddFluentAnnotations(...) with scoped and common culture and resource types:

services.AddFluentAnnotations(
    builder =>
        // Conditional Localization rule for German 
        // culture and resource type scoped to LoginDto
        builder.For<LoginDto>()
            .When(x => x.LangCode == 'DE')
            .WithCulture(CultureInfo.GetCultureInfo("de-DE"))
            .WithValidationResource<AuthenticationMessages>()
        .Build(),
    configureBehavior: options =>
    {
        // common French culture and resource type for all validation rules
        options.CommonCulture = CultureInfo.GetCultureInfo("fr-FR");
        options.CommonResourceType = typeof(ValidationMessages);
    }
);

Installation

dotnet add package FluentAnnotationsValidator --version 2.0.0-preview.2.3

Key Concepts

Test Coverage

• ✅ [Required], [EmailAddress], [MinLength], [Range], [StringLength]
• ✅ .resx and static resource support
• ✅ Record constructor annotations
• ✅ Upfront rule hydration + conditional overrides

Learn More

• GitHub Repository
• Documentation

Contribute

Help shape validation ergonomics for developers worldwide. Open to extensions, diagnostics, and new DSL patterns — bring your ideas!