.NET Performance Patterns

Scan C#/.NET code for performance anti-patterns and produce prioritized findings with concrete fixes. Patterns sourced from the official .NET performance blog series, distilled to customer-actionable guidance.

When to Use

• Reviewing C#/.NET code for performance optimization opportunities
• Auditing hot paths for allocation-heavy or inefficient patterns
• Systematic scan of a codebase for known anti-patterns before release
• Second-opinion analysis after manual performance review

When Not to Use

• Algorithmic complexity analysis — this skill targets API usage patterns, not algorithm design
• Code not on a hot path with no performance requirements — avoid premature optimization

Inputs

Input	Required	Description
Source code	Yes	C# files, code blocks, or repository paths to scan
Hot-path context	Recommended	Which code paths are performance-critical
Target framework	Recommended	.NET version (some patterns require .NET 8+)
Scan depth	Optional	critical-only, standard (default), or comprehensive

Workflow

Step 1: Load Reference Files (if available)

Try to load references/critical-patterns.md and the topic-specific reference files listed below. These contain detailed detection recipes and grep commands.

If reference files are not found (e.g., in a sandboxed environment or when the skill is embedded as instructions only), skip file loading and proceed directly to Step 3 using the scan recipes listed inline below. Do not spend time searching the filesystem for reference files — if they aren't at the expected relative path, they aren't available.

Step 2: Detect Code Signals and Select Topic Recipes

Scan the code for signals that indicate which pattern categories to check. If reference files were loaded, use their ## Detection sections. Otherwise, use the inline recipes in Step 3.

Signal in Code	Topic
async, await, Task, ValueTask	Async patterns
Span<, Memory<, stackalloc, ArrayPool, string.Substring, .Replace(, .ToLower(), += in loops, params	Memory & strings
Regex, [GeneratedRegex], Regex.Match, RegexOptions.Compiled	Regex patterns
Dictionary<, List<, .ToList(), .Where(, .Select(, LINQ methods, static readonly Dictionary<	Collections & LINQ
JsonSerializer, HttpClient, Stream, FileStream	I/O & serialization

Always check structural patterns (unsealed classes) regardless of signals.

Scan depth controls scope:

• critical-only: Only critical patterns (deadlocks, >10x regressions)
• standard (default): Critical + detected topic patterns
• comprehensive: All pattern categories

Step 3: Scan and Report

For files under 500 lines, read the entire file first — you'll spot most patterns faster than running individual grep recipes. Use grep to confirm counts and catch patterns you might miss visually.

For each relevant pattern category, run the detection recipes below. Report exact counts, not estimates.

Core scan recipes (run these when reference files aren't available):

# Strings & memory
grep -n '\.IndexOf(\"' FILE                    # Missing StringComparison
grep -n '\.Substring(' FILE                    # Substring allocations
grep -En '\.(StartsWith|EndsWith|Contains)\s*\(' FILE  # Missing StringComparison
grep -n '\.ToLower()\|\.ToUpper()' FILE        # Culture-sensitive + allocation
grep -n '\.Replace(' FILE                      # Chained Replace allocations
grep -n 'params ' FILE                         # params array allocation

# Collections & LINQ
grep -n '\.Select\|\.Where\|\.OrderBy\|\.GroupBy' FILE  # LINQ on hot path
grep -n '\.All\|\.Any' FILE                    # LINQ on string/char
grep -n 'new Dictionary<\|new List<' FILE      # Per-call allocation
grep -n 'static readonly Dictionary<' FILE     # FrozenDictionary candidate

# Regex
grep -n 'RegexOptions.Compiled' FILE           # Compiled regex budget
grep -n 'new Regex(' FILE                      # Per-call regex
grep -n 'GeneratedRegex' FILE                  # Positive: source-gen regex

# Structural
grep -n 'public class \|internal class ' FILE  # Unsealed classes
grep -n 'sealed class' FILE                    # Already sealed
grep -n ': IEquatable' FILE                    # Positive: struct equality

Rules:

• Run every relevant recipe for the detected pattern categories
• Emit a scan execution checklist before classifying findings — list each recipe and the hit count
• A result of 0 hits is valid and valuable (confirms good practice)
• If reference files were loaded, also run their ## Detection recipes

Verify-the-Inverse Rule: For absence patterns, always count both sides and report the ratio (e.g., "N of M classes are sealed"). The ratio determines severity — 0/185 is systematic, 12/15 is a consistency fix.

Step 3b: Cross-File Consistency Check

If an optimized pattern is found in one file, check whether sibling files (same directory, same interface, same base class) use the un-optimized equivalent. Flag as 🟡 Moderate with the optimized file as evidence.

Step 3c: Compound Allocation Check

After running scan recipes, look for these multi-allocation patterns that single-line recipes miss:

1. Branched .Replace() chains: Methods that call .Replace() across multiple if/else branches — report total allocation count across all branches, not just per-line.
2. Cross-method chaining: When a public method delegates to another method that itself allocates intermediates (e.g., A calls B which does 3 regex replaces, then A calls C), report the total chain cost as one finding.
3. Compound += with embedded allocating calls: Lines like result += $"...{Foo().ToLower()}" are 2+ allocations (interpolation + ToLower + concatenation) — flag the compound cost, not just the .ToLower().
4. string.Format specificity: Distinguish resource-loaded format strings (not fixable) from compile-time literal format strings (fixable with interpolation). Enumerate the actionable sites.

Step 4: Classify and Prioritize Findings

Assign each finding a severity:

Severity	Criteria	Action
🔴 Critical	Deadlocks, crashes, security vulnerabilities, >10x regression	Must fix
🟡 Moderate	2-10x improvement opportunity, best practice for hot paths	Should fix on hot paths
ℹ️ Info	Pattern applies but code may not be on a hot path	Consider if profiling shows impact

Prioritization rules:

1. If the user identified hot-path code, elevate all findings in that code to their maximum severity
2. If hot-path context is unknown, report 🔴 Critical findings unconditionally; report 🟡 Moderate findings with a note: "Impactful if this code is on a hot path"
3. Never suggest micro-optimizations on code that is clearly not performance-sensitive

Scale-based severity escalation: When the same pattern appears across many instances, escalate severity:

• 1-10 instances of the same anti-pattern → report at the pattern's base severity
• 11-50 instances → escalate ℹ️ Info patterns to 🟡 Moderate
• 50+ instances → escalate to 🟡 Moderate with elevated priority; flag as a codebase-wide systematic issue

Always report exact counts (from scan recipes), not estimates or agent summaries.

Step 5: Generate Findings

Keep findings compact. Each finding is one short block — not an essay. Group by severity (🔴 → 🟡 → ℹ️), not by file.

Format per finding:

#### ID. Title (N instances)
**Impact:** one-line impact statement
**Files:** file1.cs:L1, file2.cs:L2, ... (list locations, don't build tables)
**Fix:** one-line description of the change (e.g., "Add `StringComparison.Ordinal` parameter")
**Caveat:** only if non-obvious (version requirement, correctness risk)

Rules for compact output:

• No ❌/✅ code blocks for trivial fixes (adding a keyword, parameter, or type change). A one-line fix description suffices.
• Only include code blocks for non-obvious transformations (e.g., replacing a LINQ chain with a foreach loop, or hoisting a closure).
• File locations as inline comma-separated list, not a table. Use File.cs:L42 format.
• No explanatory prose beyond the Impact line — the severity icon already conveys urgency.
• Merge related findings that share the same fix (e.g., all .ToLower() calls go in one finding, not split by file).
• Positive findings in a bullet list, not a table. One line per pattern: ✅ Pattern — evidence.

End with a summary table and disclaimer:

| Severity | Count | Top Issue |
|----------|-------|-----------|
| 🔴 Critical | N | ... |
| 🟡 Moderate | N | ... |
| ℹ️ Info | N | ... |

> ⚠️ **Disclaimer:** These results are generated by an AI assistant and are non-deterministic. Findings may include false positives, miss real issues, or suggest changes that are incorrect for your specific context. Always verify recommendations with benchmarks and human review before applying changes to production code.

Validation

Before delivering results, verify:

• All critical patterns were checked (from reference files or inline recipes)
• Topic-specific recipes run only when matching signals detected
• Each finding includes a concrete code fix
• Scan execution checklist is complete (all recipes run)
• Summary table included at end

Common Pitfalls

Pitfall	Correct Approach
Flagging every Dictionary as needing FrozenDictionary	Only flag if the dictionary is never mutated after construction
Suggesting Span<T> in async methods	Use Memory<T> in async code; Span<T> only in sync hot paths
Reporting LINQ outside hot paths	Only flag LINQ in identified hot paths or tight loops; LINQ is acceptable in code that runs infrequently. Since .NET 7, LINQ Min/Max/Sum/Average are vectorized — blanket bans on LINQ are misguided
Suggesting ConfigureAwait(false) in app code	Only applicable in library code; not primarily a performance concern
Recommending ValueTask everywhere	Only for hot paths with frequent synchronous completion
Flagging new HttpClient() in DI services	Check if IHttpClientFactory is already in use
Suggesting [GeneratedRegex] for dynamic patterns	Only flag when the pattern string is a compile-time literal
Suggesting CollectionsMarshal.AsSpan broadly	Only for ultra-hot paths with benchmarked evidence; adds complexity and fragility
Suggesting unsafe code for micro-optimizations	Avoid unsafe except where absolutely necessary — do not recommend it for micro-optimizations that don't matter. Safe alternatives like Span<T>, stackalloc in safe context, and ArrayPool cover the vast majority of performance needs