Analyzing .NET Performance

.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

Workflow

Step 1: Load Critical Reference

Resolve bundled paths from the directory that contains this SKILL.md, not from the user's workspace. Load this reference file first:

• references/critical-patterns.md

If a direct read fails, list this skill's references/ directory once and retry only when the listing shows the expected file. Do not use workspace file or text search to locate the skill installation.

Step 2: Detect Code Signals and Select Topic Recipes

Scan the code for signals that indicate which pattern categories to check. Use the ## Detection section from the critical reference when available and the inline recipes in Step 3 for initial signal detection.

After detecting signals, load only the topic-specific references selected by scan depth:

• critical-only: No additional references (use only critical-patterns.md)
• standard (default): Load references matching detected signals from this list:
  • references/async-patterns.md — async/Task/ValueTask signals
  • references/memory-and-strings.md — Span/Memory/string allocation signals
  • references/regex-patterns.md — Regex signals
  • references/collections-and-linq.md — Dictionary/List/LINQ signals
  • references/io-and-serialization.md — JsonSerializer/HttpClient/Stream signals
  • references/structural-patterns.md — always loaded (unsealed classes checked regardless)
• comprehensive: Load all six topic-specific references above

For coverage reporting, the selected references are references/critical-patterns.md plus only the topic-specific references selected above. If any selected reference remains unavailable after retry, use the inline recipes in Step 3 for the missing coverage. Include Reference coverage: reduced; unavailable: <paths>; used inline recipes for missing references. in the final report, with <paths> replaced by the missing relative paths of the selected references only.

Use the ## Detection sections from loaded reference files and the inline recipes in Step 3 for categories whose reference files are unavailable.

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.