Dotnet Bespoke Code Minimisation

Overview

Bias against bespoke internal implementations by preferring mature open-source tools and composable libraries. When custom code is unavoidable, enforce justification rubrics that require explicit rationale, ownership, versioning, testing, and security considerations.

When to Use

• Proposals to write internal mappers, code generators, custom build scripts, or DI containers
• Reviews where new internal tooling or bespoke framework layers are introduced
• Evaluating whether to build vs buy/adopt for any infrastructure component
• Reviewing PRs that add custom implementations for common concerns (validation, retries, caching)
• Architectural decisions involving custom scaffolding or automation scripts

Core Workflow

1. Identify the concern: Determine what functionality is being proposed (mapping, validation, CLI, etc.)
2. Search for OSS alternatives: Evaluate mature open-source libraries that solve the same problem
3. Compare trade-offs: Assess maintenance burden, test effort, performance, and community support
4. Apply decision framework: Choose OSS unless clear justification exists for bespoke code
5. Require justification rubric: If bespoke code is proposed, demand explicit rationale, ownership, versioning, tests, and security considerations
6. Document decision: Record OSS evaluation, selection rationale, and maintenance plan
7. Verify in PR: Check that PR includes evidence of OSS evaluation and rubric satisfaction

Core

Defaults

• Prefer OSS libraries/tools over bespoke implementations for:
  • mapping,
  • validation,
  • retries/circuit breakers,
  • caching,
  • scheduling,
  • logging/metrics,
  • CLI/automation tools,
  • test harnesses.

Principles

• "Library before framework": small, composable components are preferred.
• "Configuration before code" when it improves transparency and reduces maintenance.
• "Script last": if unavoidable, scripts must be versioned, tested, and documented.

Load: checklists

Bespoke justification rubric (required)

A bespoke internal tool/framework must include:

• explicit rationale why OSS alternatives are insufficient,
• ownership (team/person) and support model,
• versioning and deprecation policy,
• tests and documentation,
• security and supply-chain considerations.

Load: examples

• Prefer an OSS formatter/analyzer/CLI over a custom PowerShell script.
• Prefer an OSS mapping generator over internal reflection-based mapping.

Load: enforcement

• Reject PRs introducing new internal framework layers without:
  • justification rubric satisfied,
  • a documented maintenance plan,
  • confirmation that OSS options were evaluated and licensing revalidated.

Load: PR review checklist

When reviewing PRs proposing new custom code, verify:

• OSS Evaluation: Has the author evaluated mature OSS alternatives? (Expect evidence: library list, why each was rejected)
• Justification Rubric: If OSS rejected, is the full rubric satisfied?
  • Explicit rationale for OSS insufficiency
  • Named owner (team/person) responsible for maintenance
  • Versioning and deprecation policy defined
  • Tests covering critical functionality
  • Security and supply-chain considerations documented
• Library vs Framework: For applicable cases, has the author chosen libraries over frameworks?
• Documentation: Is maintenance burden and support model clearly communicated?
• Red Flags: Watch for rationalizations that bypass evaluation (e.g., "we need full control", "OSS is too heavy", "we're special")

Decision rule: If OSS evaluation is incomplete or rubric unsatisfied, request changes before approving.

Load: worked example

Scenario: Object Mapping in .NET

Requirement: Convert DTOs to domain entities in a health insurance claims system.

Option A: Custom Reflection-Based Mapper (Bespoke)

public class ClaimMapper
{
    public DomainClaim MapToClaim(ClaimDto dto)
    {
        var claim = new DomainClaim();
        // Manual property assignment for ~30 properties
        claim.ClaimId = dto.Id;
        claim.MemberId = dto.MemberId;
        // ... 28 more assignments
        return claim;
    }
}

Costs:

• Maintenance: Manual updates required when entities change (tight coupling)
• Testing: Every mapping path must be tested manually
• Performance: Reflection-based or slow property copying
• Versioning: No clear deprecation path if mapping rules change
• Ownership: Who maintains this when the original author leaves?

OSS Evaluation: Rejected without justification.

Option B: AutoMapper (OSS Library)

services.AddAutoMapper(cfg =>
{
    cfg.CreateMap<ClaimDto, DomainClaim>();
});

Strengths:

• Maintenance: Configuration-driven, auto-discovers properties by name/convention
• Testing: Industry-standard test patterns, extensive test suite in OSS
• Performance: Mature optimization, benchmarked at scale
• Versioning: Library follows SemVer; breaking changes documented
• Ownership: Active maintainers, funding model established
• Documentation: Comprehensive guides for complex mappings

Risks: Dependency on external library (mitigated by extensive industry adoption and source availability).

Option C: Mapperly (Modern OSS Library)

[Mapper]
public partial class ClaimMapper
{
    public partial DomainClaim MapToClaim(ClaimDto dto);
}

Strengths:

• Zero runtime overhead via source generation (better than AutoMapper for performance-critical paths)
• Explicit, generated code is auditable
• Compile-time safety
• Minimal dependencies
• Fastest execution path

Trade-off: Newer library (active development but smaller ecosystem than AutoMapper).

Decision Framework

Recommendation

Use Mapperly for new systems (source generation, zero deps, best performance) or AutoMapper for teams with existing expertise.

Reject bespoke mapper unless:

• Performance benchmarks prove custom code materially faster (at scale)
• Mapping logic is genuinely bespoke (not property-to-property)
• Ownership, versioning, and testing documented per rubric

Verification

Evidence required in PR:

• OSS libraries evaluated: AutoMapper, Mapperly, TinyMapper considered
• Selection rationale: "Mapperly chosen for source-gen performance and zero-dependency model"
• Tests: Core mappings covered
• Documentation: Mapping conventions explained (if non-obvious)

Red Flags - STOP

These statements indicate bypass of bespoke minimisation principles:

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