Architecture Overview

1. Core Design Philosophy

FluentAnnotationsValidator is built around a type-safe, override-safe validation engine that dynamically translates [ValidationAttribute] annotations and fluent rule definitions into runtime validation logic. Although in its early development process, it represents an alternative to the FluentValidation by offering its own fluent DSL, conditional logic, and localization support.

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2. Key Components

Layer	Purpose
Metadata	Handles [ValidationResource] annotations and resource resolution for localized messages.
Configuration	Hosts the fluent DSL surface (Rule, RuleFor, Must, When, etc.), rule builders, and behavior options.
Runtime.Validators	Executes validation logic for both attribute-based and fluent-defined rules. Includes custom validators like FluentLengthAttribute, EqualAttribute, etc.
Internals.Reflection	Caches metadata and member info for performance. Powers dynamic rule resolution and override detection.
Messages	Manages localization, formatting, and fallback logic for error messages. Supports .resx and static resource classes.
Results	Defines validation result types and error structures returned during runtime validation.
Extensions	Provides service registration methods like AddFluentAnnotations(...) for ASP.NET Core integration.

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3. Validation Flow

1. Model Discovery
   Types are scanned for [ValidationAttribute] annotations, IFluentValidatable implementations, or registered via .For<T>().

2. Rule Registration
   Rules are attached using either:

   • Static attributes ([Required], [EmailAddress], etc.)
   • Fluent DSL (Rule(x => x.Password).Required().MinimumLength(6).MaximumLength(20), RuleFor(x => x.Email).NotEmpty().EmailAddress())

3. Conditional Logic
   Rules can be grouped under When(...) and Otherwise(...) blocks for dynamic evaluation.

4. Localization
   Error messages are resolved using:

   • Conventional keys (Property_Attribute)
   • Explicit resource keys
   • Fallback messages

5. Execution
   At runtime, IFluentValidator<T> validates the model, applying all applicable rules and returning structured results.

┌─────────────────────────────┐
│     1. Model Discovery      │
│  (Scan types, attributes)   │
└─────────────┬───────────────┘
              │
              ▼
┌─────────────────────────────┐
│     2. Rule Registration    │
│  (Attributes + Fluent DSL)  │
└─────────────┬───────────────┘
              │
              ▼
┌─────────────────────────────┐
│  3. Conditional Logic       │
│  (`When`/`Otherwise`)       │
└─────────────┬───────────────┘
              │
              ▼
┌─────────────────────────────┐
│     4. Localization         │
│  (Resolve error messages)   │
└─────────────┬───────────────┘
              │
              ▼
┌─────────────────────────────┐
│     5. Execution            │
│  (Run rules, return results)│
└─────────────────────────────┘

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4. Extensibility Points

• IFluentValidator<T>: Custom validators can be injected and extended.
• ConditionalValidationRule: Supports per-rule predicates and override logic.
• BeforeValidation(...): Allows pre-validation value transformation or initialization.
• ValidationBehaviorOptions: Centralized configuration for culture, resource types, and rule override behavior.