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.agents/skills/analyzing-dotnet-performance/SKILL.md

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+---
+name: analyzing-dotnet-performance
+description: >-
+  Scans .NET code for ~50 performance anti-patterns across async, memory,
+  strings, collections, LINQ, regex, serialization, and I/O with tiered
+  severity classification. Use when analyzing .NET code for optimization
+  opportunities, reviewing hot paths, or auditing allocation-heavy patterns.
+---
+
+# .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:
+
+```markdown
+| 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 |

.agents/skills/analyzing-dotnet-performance/references/async-patterns.md

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+# Async & Concurrency Patterns
+
+### Don't Expose Async Wrappers for Sync Methods
+🟡 **AVOID** wrapping sync methods with `Task.Run` in libraries | .NET Core+
+
+❌
+```csharp
+public Task<int> ComputeHashAsync(byte[] data) =>
+    Task.Run(() => ComputeHash(data));
+```
+✅
+```csharp
+public int ComputeHash(byte[] data) { /* CPU-bound work */ }
+// Consumer decides: var hash = await Task.Run(() => lib.ComputeHash(data));
+```
+
+**Impact: Eliminates unnecessary thread pool queue/dequeue overhead per call.**
+
+### Don't Expose Sync Wrappers for Async Methods
+🟡 **AVOID** creating sync wrappers that block on async implementations | .NET Core+
+
+❌
+```csharp
+public string GetData() => GetDataAsync().Result;
+```
+✅
+```csharp
+public async Task<string> GetDataAsync() { /* ... */ }
+```
+
+**Impact: Prevents deadlocks and thread pool starvation from hidden sync-over-async blocking.**
+
+### Use ValueTask for Hot Paths with Frequent Sync Completion
+🟡 **DO** use `ValueTask<T>` on hot paths where sync completion is common | .NET Core 2.1+
+
+❌
+```csharp
+public async Task<int> ReadAsync(Memory<byte> buffer)
+{
+    if (_bufferedCount > 0)
+        return ReadFromBuffer(buffer.Span);
+    return await ReadAsyncCore(buffer);
+}
+```
+✅
+```csharp
+public ValueTask<int> ReadAsync(Memory<byte> buffer)
+{
+    if (_bufferedCount > 0)
+        return new ValueTask<int>(ReadFromBuffer(buffer.Span));
+    return new ValueTask<int>(ReadAsyncCore(buffer));
+}
+```
+
+**Impact: Eliminates Task\<T\> allocation on synchronous completion — the struct stores results inline.**
+
+### Use Channels for Producer/Consumer
+🟡 **DO** use `System.Threading.Channels` for producer-consumer patterns | .NET Core 3.0+
+
+❌
+```csharp
+var queue = new BlockingCollection<WorkItem>();
+var item = queue.Take();
+```
+✅
+```csharp
+var channel = Channel.CreateUnbounded<WorkItem>();
+
+// Producer
+await channel.Writer.WriteAsync(item);
+
+// Consumer
+await foreach (var item in channel.Reader.ReadAllAsync())
+    Process(item);
+```
+
+**Impact: ~25% faster, ~95% fewer GC collections vs manual approaches.**
+
+### Avoid False Sharing with Thread-Local State
+🟡 **AVOID** adjacent mutable fields written by different threads | .NET 7+
+
+❌
+```csharp
+class SharedCounters
+{
+    public long Counter1;
+    public long Counter2;
+}
+```
+✅
+```csharp
+[StructLayout(LayoutKind.Explicit, Size = 128)]
+struct PaddedCounter
+{
+    [FieldOffset(0)] public long Value;
+}
+```
+
+**Impact: Eliminates cross-core cache invalidation — can improve multi-threaded throughput by 10x+.**
+
+## Detection
+
+Scan recipes for async anti-patterns. Run these and report exact counts.
+
+```bash
+# async void methods (correctness issue — crashes on exception)
+grep -rn --include='*.cs' 'async void' --exclude-dir=bin --exclude-dir=obj . | grep -v 'event' | wc -l
+```
+
+### Patterns Requiring Manual Review
+
+- **Sync-over-async** (`.Result`, `.Wait()`): `.Result` matches any property named Result — needs type context to confirm it's `Task.Result`