System.ComponentModel.DataAnnotations Validation — Maintainer's Guide

This repository contains a comprehensive guide to .NET's System.ComponentModel.DataAnnotations validation system. It covers everything from built-in attributes through advanced extensibility, and explores the path toward adding async validation to DataAnnotations. The material is designed to bring a team up to speed so they can build a project plan for adding async validation across the entire .NET product suite.

Chapters

1. Built-In Validation Attributes — Complete catalog of attributes shipped with .NET
2. Creating Custom Validation Attributes — Deriving from ValidationAttribute for reusable rules
3. Annotating Objects for Validation — Applying attributes to properties and classes
4. Programmatic (Manual) Validation — Using the Validator class directly
5. ASP.NET MVC Automatic Validation — How model binding triggers validation
6. Advanced Custom Validation — IValidatableObject and complex validation scenarios
7. ValidationContext Deep Dive — Services, Items dictionary, and context propagation
8. The ValidationResult API — Understanding validation results and error reporting
9. The Async Validation Gap — Why DataAnnotations lacks async support today
10. Strickland — Parallel Concepts and Async Validation — Lessons from a JavaScript validation framework
11. The History of DataAnnotations Integration Across .NET — Chronological history of every integration point
12. The Async Validation Prototype — A Working Demo — Analysis of a working async validation prototype
13. Object Graph Validation — Recursive walks, result representation, and ecosystem solutions

Appendices

• Appendix A: .NET Integration Points Catalog — Every framework that consumes DataAnnotations
• Appendix B: Complete Reference Link Library — All source links and documentation references

Suggested Reading Order

For team members who are new to DataAnnotations validation, the recommended reading order below starts with the foundational blog series, moves through official API documentation, then application model integration, and finishes with async validation concepts.

Phase 1: Historical Foundation (RIA Services Blog Series)

DataAnnotations validation saw its most significant innovation during the .NET RIA Services era (2009–2012), when APIs like IValidatableObject, ValidationResult, ValidationContext, and Validator were designed and refined. The 2010 RIA Services Validation blog series was written against RIA Services but covers the universal DataAnnotations framework, and it remains one of the most thorough walkthroughs available.

1. Standard Validators — [Required], [Range], [StringLength], [RegularExpression]; error messages and localization
2. Custom Validation Methods — Using [CustomValidation] to delegate to static methods
3. Custom Reusable Validators — Deriving from ValidationAttribute; the IsValid(object, ValidationContext) override
4. Attribute Propagation — How validators flow between tiers
5. Validation Triggers — When validation is invoked
6. Cross-Field Validation — Validating one property based on another
7. Entity-Level Validation — Class-level validation and the 4-stage pipeline
8. Providing ValidationContext — Items dictionary, IServiceProvider, ServiceContainer
9. Using ValidationContext (Cross-Entity Validation) — Service-based validators that access data stores
10. ViewModel Validation with Entity Rules — Reusing validation across view models

Additional Resources:

• GitHub repository with sample code: jeffhandley/RIAServicesValidation
• Video tutorials (VB): RIA Services Validation Video Tutorials

Phase 2: Modern .NET API Documentation

11. System.ComponentModel.DataAnnotations Namespace
12. Validator Class API
13. ValidationAttribute Class
14. ValidationContext Class
15. ValidationResult Class

Phase 3: Application Model Integration

16. ASP.NET Core MVC Model Validation
17. Blazor Forms Validation
18. Options Pattern Validation — ValidateDataAnnotations(), ValidateOnStart()
19. EF Core Data Annotations
20. ASP.NET Core OpenAPI
21. WPF Data Validation
22. WinForms Input Validation

Phase 4: Async Validation Concepts

23. Strickland Documentation / GitHub
24. Strickland: Inspiration
25. Strickland: Two-Stage Sync/Async Validation

Phase 5: Integration History and Working Prototype

26. Chapter 11: The History of DataAnnotations Integration Across .NET — Chronological history of every integration point with PR/issue citations
27. Chapter 12: The Async Validation Prototype — A Working Demo — Analysis of the oroztocil/validation-demo branch
28. Microsoft.Extensions.Validation design issue (dotnet/aspnetcore#46349) — Unified validation design proposal
29. Minimal API validation PR (dotnet/aspnetcore#60724) — Core implementation of Minimal API validation

About This Material

• Core source analysis — dotnet/runtime, dotnet/aspnetcore, dotnet/efcore, and dotnet/aspire repositories, tracing validation integration history through PRs and commits
• Historical foundation — The RIA Services Validation blog series (2010) and research into how RIA Services and OpenRiaServices approached async validation patterns
• Official documentation — Microsoft Learn conceptual and API docs for ASP.NET Core MVC, Blazor, Options pattern, EF Core, OpenAPI, WPF, and WinForms validation
• Async validation prototype — Analysis of the oroztocil/validation-demo branch and the Microsoft.Extensions.Validation design
• Ecosystem research — Strickland (JavaScript validation framework), MiniValidation (Damian Edwards), FluentValidation, Blazor component vendors (MudBlazor, Radzen, Telerik, Syncfusion, DevExpress), API frameworks (FastEndpoints, ServiceStack), and schema generators (Swashbuckle, NSwag)
• The material targets team members who will work on adding async validation across the .NET product suite

License

This repository is licensed under the MIT License. See the LICENSE file for details.