Prompt Engineering

Modernize C# Code — Current Language, Identical Behavior

Pattern matching, records, nullable annotations, async done right — a C# modernization scope with the rule that matters: modern equivalents are rarely exact equivalents.

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Overview

C# has changed enough that pre-C# 8 code reads like a different language — but modernization is a behavior minefield: LINQ changes evaluation laziness, async changes timing, records change equality semantics. This prompt sets a concrete C# modernization scope: pattern matching and switch expressions over conditional chains, records for immutable data carriers, nullable reference annotations where the project enables them, LINQ only where it stays readable, async/await over raw continuations and never blocking with .Result. The modernization goal's warning does the safety work: check null handling, error behavior, and lazy-vs-eager evaluation before swapping anything.

How to use this resource

Scope the features

A named list — patterns, records, nullable annotations, async hygiene — not a vague "make it modern".
Swap with suspicion

Each replacement is checked against documented behavior differences: null handling, laziness, equality.
Verify edge cases per API

Old and new calls must agree where it hurts — empty sequences, nulls, exceptions — not just the happy path.

Why This Works

A concrete feature scope produces consistent modernization instead of taste
The not-exact-equivalents warning targets where modernizations actually regress
Per-replacement verifiability keeps the diff reviewable

Best for

.NET codebases upgraded by framework but not by idiom
Teams adopting nullable reference types incrementally
Code written before records and switch expressions existed

Not for

Reviewing C# code quality without transforming it — that's the Code Review Prompt Generator
Optimizing the SQL inside the C# — that's the SQL Optimization Prompt's space

Use cases

Bringing a C# 7-era service class to current C#
Replacing conditional chains with pattern matching
Untangling .Result blocking from async call paths

FAQ

Will modernizing to records or LINQ quietly change how my C# behaves?

The prompt is built to prevent that. Its known-failure-mode warning says "modern equivalents are rarely exact equivalents — check null handling, error behavior, lazy vs eager evaluation, and culture/locale defaults before swapping," and validation verifies each API replacement against documented edge cases. "If achieving the goal and preserving behavior conflict, behavior wins." It generates the refactor prompt; you run it and regression-test the diff yourself.

What if part of my C# can't be safely modernized from what I paste in?

It leaves that code alone and says why. The objective reads "If a transformation cannot be made safely with the available information, leave that code unchanged and say why," and skipped transformations land under "Suggested but not applied" with the information that would unlock them. Blocking on async with .Result, for instance, is only untangled when the call graph is clear. You review before applying.

Customize This Resource

Opens this setup in Refactor Prompt Builder. Generate to get the full refactoring contract — then adjust the goal, risk level, and validation strategy.

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Prompt Template

Copy it as-is, or use Open in Refactor Prompt Builder to load it pre-filled and customize it with your own context.

REFACTORING OBJECTIVE
Modernize this C# 7-era service class to current C# without changing its behavior.
Refactoring goal: modernization — move the code to current language features and supported APIs, behavior intact.
Change how the code is built — never what the code does. If a transformation cannot be made safely with the available information, leave that code unchanged and say why.

EXISTING CODE CONTEXT
Code context: none specified — treat unknown consumers as possible and unstated requirements as existing.
Language: C# — keep the refactored code idiomatic and consistent with the file's existing style.
Code to refactor:
```
[Paste the code to refactor here]
```

REFACTORING GOAL
Primary goal: Modernization.
Transformation priorities for this goal:
1. Replace deprecated and obsolete APIs with their supported successors.
2. Adopt current language features where they make the code simpler — not everywhere they merely fit.
3. Update outdated patterns — manual loops, callback pyramids, hand-rolled utilities — to their modern equivalents.
4. Modernize incrementally: each replacement should be independently verifiable.

RISK PROFILE
Risk level: Balanced.
- Apply reasonable improvements: moderate restructuring is allowed where the benefit is clear.
- Group related changes, but keep each group reviewable and separately verifiable.
- When a transformation is borderline-safe, apply the conservative version and note the more aggressive option.

SAFETY RULES
These rules apply to every transformation, at every risk level:
1. Identify behavior assumptions before changing code — list every assumption you rely on.
2. Flag uncertain behavior instead of deciding it: if you cannot tell what a branch does or why it exists, ask — do not guess.
3. Do not remove functionality. Code that looks redundant may be load-bearing.
4. Do not invent requirements. Refactor toward the stated goal, not toward an imagined better product.
5. Do not rewrite unrelated code. Touch only what the goal requires.
6. Explain why each change exists — every transformation traces to the stated goal or to a named problem in the code.
7. Preserve public contracts — signatures, return types, error types, serialized shapes — unless explicitly instructed otherwise.

TRANSFORMATION GUIDANCE
Known failure modes of this goal — watch for:
- Modern equivalents are rarely exact equivalents — check null handling, error behavior, lazy vs eager evaluation, and culture/locale defaults before swapping.
- Do not modernize for fashion: a working pattern with no maintenance cost may not be worth the regression risk of replacing it.
Modernization scope for C#:
- Use pattern matching and switch expressions where they replace conditional chains.
- Use records for immutable data carriers; use init-only setters where mutation is not needed.
- Adopt nullable reference type annotations where the project enables them.
- Replace manual loops with LINQ only where it stays readable — LINQ is not a goal in itself.
- Use async/await over raw Task continuations; never block on async code with .Result or .Wait().

BEHAVIOR PRESERVATION REQUIREMENTS
- Preserve observable behavior: the same inputs produce the same outputs, including error cases.
- Preserve business rules: every condition, threshold, and special case survives the refactor exactly.
- Preserve outputs: formats, ordering, precision, and encodings stay identical unless the goal explicitly targets them.
- Preserve side effects: writes, emitted events, log lines other systems consume — and their relative order.
- Preserve integrations: API shapes, serialized payloads, database access patterns, and external call sequences.
- These requirements hold unless the instructions explicitly say otherwise — and any knowingly behavior-affecting change must be flagged, never silent.
- If achieving the goal and preserving behavior conflict, behavior wins: stop and explain the conflict instead of compromising it.

VALIDATION STRATEGY
Goal-specific validation: verify each API replacement against its documented behavior differences — old and new must agree on edge cases, not just the happy path.
- Identify the key workflows this code serves and describe how to verify each still works after the refactor.
- List the major edge cases the original code handles and confirm each is still handled — name them individually.
- Recommend regression checks for the riskiest transformations made, riskiest first.

ASSUMPTIONS
- List every assumption made about the code's behavior, inputs, or environment.
- Mark each assumption VERIFIED (with its evidence) or UNVERIFIED (with the question that would resolve it).
- Any transformation that depends on an UNVERIFIED assumption must be flagged as conditional on it.

NON-GOALS
- Do not invent requirements.
- Do not redesign the entire system.
- Do not remove behavior without justification.
- Do not introduce new features.
- Separate assumptions clearly from facts.
- Flag uncertainty instead of resolving it silently.
- Explain the tradeoffs of any change that has them.

OUTPUT REQUIREMENTS
- Return the refactored code, complete — no elided sections, no "rest unchanged" placeholders inside changed units.
- Follow it with a change list: for each change — what changed, why it serves the goal, and its behavior impact (expected: none).
- Flag any change that could plausibly alter behavior, with the reason it might and the check that would confirm it does not.
- Where a worthwhile transformation was skipped for safety, list it under "Suggested but not applied" with what information would unlock it.

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