From dotnet-diag
Scans C#/.NET code for 50+ performance anti-patterns in async, memory, strings, collections, LINQ, regex, serialization, and I/O with severity tiers and fixes. Use for hot path reviews or allocation audits.
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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.
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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.
| 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 |
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.
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 patternscomprehensive: All pattern categoriesFor 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:
## Detection recipesVerify-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.
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.
After running scan recipes, look for these multi-allocation patterns that single-line recipes miss:
.Replace() chains: Methods that call .Replace() across multiple if/else branches — report total allocation count across all branches, not just per-line.+= with embedded allocating calls: Lines like result += $"...{Foo().ToLower()}" are 2+ allocations (interpolation + ToLower + concatenation) — flag the compound cost, not just the .ToLower().string.Format specificity: Distinguish resource-loaded format strings (not fixable) from compile-time literal format strings (fixable with interpolation). Enumerate the actionable sites.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:
Scale-based severity escalation: When the same pattern appears across many instances, escalate severity:
Always report exact counts (from scan recipes), not estimates or agent summaries.
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:
File.cs:L42 format..ToLower() calls go in one finding, not split by file).✅ 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.
Before delivering results, verify:
| 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 |