Best Way to Implement Letter-Boxed Style Validation Logic in an App?

For a Letter-Boxed–style game, validation logic should be implemented on both client and server, with different responsibilities.

1. Where to validate

• Client-side (C#, .NET MAUI, Blazor, or JavaScript/TypeScript)
  • Validate quickly for UX: highlight invalid letters, disallow illegal transitions, and show immediate error messages while the user types or moves focus.
  • This is analogous to validating view models or input fields in the presentation layer:
    • In .NET MAUI, use a ValidatableObject<T>-style wrapper around the user’s current word or word list, with rules like:
      • “All characters must be from the allowed letter set.”
      • “No two consecutive letters from the same side.”
      • “Word N must start with the last letter of word N-1.”
    • In web UI (Razor/ASP.NET or SPA), use client-side validation helpers or JavaScript to enforce these rules as the user edits.
  • This improves usability by avoiding round trips for obvious errors.
• Server-side (.NET API, MVC, minimal APIs)
  • Always revalidate submitted moves/solutions on the server before accepting them, to protect against tampering or bypassed client logic.
  • Treat the game commands (e.g., SubmitSolutionCommand) or DTOs as input models and validate them as part of the domain logic.
  • This follows the guidance that even with client-side validation, server-side validation is mandatory for security and correctness.

2. Recommended design approach in .NET

• Domain model as source of truth
  • Model the puzzle as a domain entity/value object (e.g., Puzzle, Side, WordSequence).
  • Implement core rules in domain services or methods:
    • bool IsValidWord(string word)
    • bool IsValidTransition(string previousWord, string nextWord)
    • bool IsValidSolution(IEnumerable<string> words)
  • This keeps the rule set centralized and testable.
• View model / DTO validation for client-side
  • In MVVM (e.g., .NET MAUI):
    • Wrap user input in ValidatableObject<string> or ValidatableObject<IList<string>>.
    • Attach validation rules that call into the domain logic or reimplement the same rules for immediate feedback.
    • Bind IsValid and Errors to the UI to highlight invalid entries and show messages.
  • In ASP.NET Web Pages / MVC / Razor:
    • Use client-side validation for form fields (e.g., words list) so errors appear as soon as the user leaves a field.

3. Client-side validation patterns (web)

• For Razor/ASP.NET Web Pages:
  • Use the built-in validation helpers and unobtrusive validation:
    • Include jQuery and jQuery Validate + unobtrusive scripts.
    • Use helpers that emit data-val-* attributes for client-side checks.
  • This gives immediate feedback while still revalidating on the server when the form is submitted.

4. Security and robustness

• Treat all user input as untrusted:
  • Validate and filter all string parameters and model properties on the server.
  • For any API endpoints that accept primitive types or raw strings (e.g., a list of words), validate them using allow-list rules (e.g., only A–Z, max length, etc.).
• Use regular expressions carefully if used for letter/word checks, and set a regex timeout if complex patterns are involved.

5. Practical split of responsibilities

• Client-side
  • Enforce syntactic and UX rules: allowed characters, no same-side adjacency, chaining between words, non-empty words, max length.
  • Provide immediate visual feedback.
• Server-side
  • Re-run all the above rules.
  • Enforce additional game-wide rules: dictionary lookup, uniqueness of words, puzzle completion criteria, scoring.
  • Persist game state and prevent cheating.

This pattern—domain model as the source of truth, with validation both in the presentation layer for UX and in the server for security and integrity—matches recommended practices for .NET applications.

References:

• Client-side validation (validation in the presentation layers)
• Validating User Input in ASP.NET Web Pages (Razor) Sites
• Validation
• Security Frame: Input Validation | Mitigations
• Overview of how to validate user input

Thank you for reaching out.

The best approach is to keep the puzzle rules in one reusable validation component (a small “validator” class/service) that takes the current game state + the user’s word and returns valid/invalid with a reason. This keeps your UI simple, makes the rules easy to unit test, and avoids duplicating logic in multiple places. A façade/aggregate-style wrapper over the rule checks is a clean pattern here (one public Validate(...) call, internally running smaller rule checks).

For where to run validation: treat server-side as the source of truth and always re-validate there, because user input is untrusted and should be validated at the trust boundary (“fail fast” on invalid input). You can still do client-side validation for instant feedback (better UX), but it should be “helpful hints” only, not the final authority. This matches the common approach in ASP.NET where validation can happen on both client and server, with the server remaining authoritative.

If you want “shared” validation between client and server, the key idea is still the same: keep the rules centralized so you don’t drift. In .NET, that usually means the rules live in a .NET library used by the server, and you optionally mirror lightweight checks on the client for responsiveness.

Helpful references:

• ASP.NET Core validation (client + server):
• Input validation guidance (treat input as untrusted / fail fast):
• https://cheatsheetseries.owasp.org/cheatsheets/Input_Validation_Cheat_Sheet.html

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