csharp-performance

name	csharp-performance
description	Guides advanced .NET performance optimization: Span<T> and Memory<T> usage, BenchmarkDotNet, allocation profiling with dotMemory, source generators, and high-performance patterns. Use when the user asks about C# performance, Span, Memory, BenchmarkDotNet, allocation profiling, source generators, high-performance .NET. Do NOT use when the user asks about C# async (use `csharp-async-patterns`), C# modern idioms (use `csharp-modern-idioms`), general performance testing (use `performance-testing`).
license	Apache-2.0
metadata	{"author":"foundry-skills","version":"1.0.0","tags":"csharp optimization debugging","category":"software-engineering","subcategory":"languages-runtimes","depends":"","disclaimer":"none","difficulty":"advanced"}

C# Performance Optimization

When to Use

Use this skill when the user:

• Asks about reducing heap allocations in C# -- including boxing, closure captures, LINQ overhead, or excessive new expressions
• Wants to use Span<T>, Memory<T>, ReadOnlySpan<T>, or ArrayPool<T> to avoid heap pressure in hot paths
• Needs to set up BenchmarkDotNet to measure method-level throughput, memory allocations, or JIT behavior
• Asks about profiling tools -- dotMemory, PerfView, dotTrace, EventPipe, or dotnet-counters -- to find allocation hotspots or GC pressure
• Wants to use source generators, Unsafe, MemoryMarshal, or NativeMemory for ultra-low-overhead patterns
• Is hitting GC pauses (Gen2 collections, LOH fragmentation) and needs to redesign object lifecycles
• Asks about value types, ref struct, readonly struct, struct layout, or [StructLayout] packing
• Wants to optimize string handling -- StringPool, SearchValues<T>, Regex compilation, or CompositeFormat
• Asks about SIMD via System.Numerics.Vector<T> or System.Runtime.Intrinsics for data-parallel computation
• Is writing library code targeting netstandard2.1 or net8+ and needs to understand API availability trade-offs

Do NOT use this skill when:

• The user asks about async/await, ValueTask, IAsyncEnumerable, or ConfigureAwait -- use csharp-async-patterns
• The user asks about C# 10--13 language features, pattern matching, records, or init-only setters in general -- use csharp-modern-idioms
• The user needs load testing, k6, Gatling, or HTTP-level throughput testing -- use performance-testing
• The user asks about EF Core query optimization, N+1 queries, or compiled queries -- use efcore-performance
• The user is asking about general algorithm complexity theory without a C# implementation context -- use a general algorithms skill
• The user asks about containerization, Kubernetes resource limits, or cloud scaling -- those are infrastructure concerns

---

Process

Step 1: Establish a Profiling-First Foundation

Before writing a single line of optimized code, build a measurement baseline.

• Select the right tool for the bottleneck type:

  • Allocation profiling: JetBrains dotMemory, or dotnet-gcdump + PerfView for allocation call trees
  • CPU hotspot profiling: JetBrains dotTrace (Timeline mode), PerfView (CPU stacks), or perf on Linux with dotnet-trace
  • GC diagnostics: dotnet-counters monitor --counters System.Runtime to watch gen-0-gc-count, gen-1-gc-count, gen-2-gc-count, loh-size, and alloc-rate in real time
  • Quick micro-benchmarks: BenchmarkDotNet with [MemoryDiagnoser] and [DisassemblyDiagnoser]
  • Production telemetry: EventPipe via DiagnosticPort or OpenTelemetry metrics with System.Runtime meters

• Run the application under a realistic workload -- never profile with toy data. For web services, replay production traffic via a captured HTTP archive or use a load generator targeting p95 latency scenarios.

• Capture a baseline snapshot before ANY change. In dotMemory: take a "Get Snapshot" after stabilization. In PerfView: collect a .etl file for 30--60 seconds under load. In BenchmarkDotNet: commit the baseline results to version control as a .csv or use --exporters json.

• Identify the top-3 allocation sites by bytes retained, not just bytes allocated. Allocated bytes tell you about throughput pressure; retained bytes identify leaks and LOH survivors.

• Check the GC mode. Server GC (<GarbageCollectionAdaptationMode>) allocates per-logical-core heaps and is nearly always correct for ASP.NET Core services. Workstation GC is correct for desktop or CLI tools. Mixing them causes unexpected pause behavior.

---

Step 2: Classify the Bottleneck with a Decision Framework

Once profiling data exists, classify the root cause before selecting an optimization.

• Allocation pressure / GC churn: Gen0 collection rate > 10/sec in a server app, or alloc-rate consistently above 100 MB/s with no corresponding throughput -- reduce allocations, pool objects, use Span<T>.

• LOH fragmentation: Objects >= 85,000 bytes land on the Large Object Heap and are not compacted by default. Symptoms: loh-size growing monotonically, gen-2-gc-count spiking. Fix: pool large buffers via ArrayPool<byte>.Shared, or enable LOH compaction with GCSettings.LargeObjectHeapCompactionMode = GCLargeObjectHeapCompactionMode.CompactOnce before a single critical GC.

• CPU hotspot in tight loop: If a single method consumes >5% of CPU samples and is called millions of times, consider: algorithmic improvement first, then Span<T> to remove bounds checks, then SIMD vectorization, then unsafe pointer arithmetic as a last resort.

• String allocation dominance: Strings are immutable reference types. If string operations dominate allocations, evaluate: string.Create<TState>(), StringBuilderPool, ReadOnlySpan<char> slicing instead of Substring, SearchValues<char> for membership tests, and CompositeFormat (net8+) to avoid repeated format string parsing.

• Reflection or dynamic dispatch overhead: If profiling shows Invoke, GetMethod, CreateInstance, or interface dispatch in hot paths, replace with source generators, cached delegates, MethodTable tricks via Unsafe.As, or DynamicMethod-based IL emit (only if source generators are not viable).

• Contention / false sharing: If CPU utilization is high but throughput is low and you see lock contentions in profiling, check: lock striping, ConcurrentDictionary, Interlocked operations, and cache-line-aligned struct layout using [StructLayout(LayoutKind.Explicit)] with 64-byte padding fields.

---

Step 3: Reduce Allocations with Span, Memory, and Pooling

This is the highest-leverage optimization category in modern .NET.

• Span<T> basics: Span<T> is a ref struct that wraps a contiguous region of memory -- stack, heap, or native. Because it is a ref struct, it cannot be boxed, stored on the heap, or used across await points. Use it for synchronous, stack-confined processing of arrays, strings (ReadOnlySpan<char>), or stack-allocated memory (stackalloc).

• stackalloc thresholds: Stack space is ~1 MB on most OS thread configurations. A practical safe limit for stackalloc is 256--1024 bytes for value types. For anything larger, fall back to ArrayPool<T>.Shared.Rent(size). Always use the pattern:

  const int StackAllocThreshold = 256;
  byte[]? pooled = null;
  Span<byte> buffer = size <= StackAllocThreshold
      ? stackalloc byte[size]
      : (pooled = ArrayPool<byte>.Shared.Rent(size));
  try { /* use buffer */ }
  finally { if (pooled is not null) ArrayPool<byte>.Shared.Return(pooled); }
  

• Memory<T> for async contexts: When you need to pass a buffer across an await boundary, use Memory<T> or ReadOnlyMemory<T>. These are regular structs (not ref struct) that wrap the same contiguous memory. Call .Span to get a Span<T> inside a synchronous scope.

• ArrayPool<T> correctness rules: Always Return the rented array even on exception (use try/finally). Never use a returned array -- zero it first with clearArray: true if it contains sensitive or stale data. Never cache views of a rented array beyond the return call.

---

Step 4: Benchmark with BenchmarkDotNet Correctly

BenchmarkDotNet is the standard C# micro-benchmarking framework. Misuse produces misleading results.

• Minimum viable benchmark setup:

  [MemoryDiagnoser]
  [DisassemblyDiagnoser(maxDepth: 3)]
  [SimpleJob(RuntimeMoniker.Net80)]
  public class ParseBenchmarks
  {
      private readonly string _input = new string('a', 1024);
  
      [Benchmark(Baseline = true)]
      public int SubstringParse() => ParseWithSubstring(_input);
  
      [Benchmark]
      public int SpanParse() => ParseWithSpan(_input.AsSpan());
  }
  

  Run with dotnet run -c Release --project Benchmarks. Never run benchmarks in Debug configuration -- the JIT does not optimize debug builds and results are meaningless.

• [Params] for realistic size coverage:

  [Params(16, 256, 4096, 65536)]
  public int InputSize { get; set; }
  

  Always include at least one small size (fits in L1 cache, ~32 KB), one medium (fits in L2/L3, ~256 KB--8 MB), and one large size (exceeds cache, forces memory bandwidth). Performance characteristics often invert across these tiers.

• [GlobalSetup] for pre-warming: Allocate test data in [GlobalSetup] to exclude setup cost from measurements. Never allocate inside [Benchmark] methods unless allocation is the thing being measured.

• Interpret [MemoryDiagnoser] output: The Allocated column shows bytes allocated per operation (not total). A benchmark showing means no managed heap allocation occurred -- this is the target for -based hot paths. , , columns show GC collections per 1000 operations.

---

Step 5: Eliminate JIT and Runtime Overhead with Advanced Patterns

After allocation reduction, these techniques address CPU-level costs.

• readonly struct: Declare structs readonly when all fields are readonly. This allows the JIT to pass the struct by reference internally without defensive copies. Without readonly, any call to a non-readonly method on a struct copied from a readonly field or in parameter triggers a silent defensive copy.

• in parameters for large structs: Passing a struct larger than 16 bytes by value copies it. Use in to pass by readonly reference: void Process(in LargeStruct s). Combine with ref readonly returns to avoid copies on return paths.

• ref returns and ref locals: Enables zero-copy access to array elements or struct fields:

  ref int element = ref data[index]; // no copy
  element += delta;                  // modifies in-place
  

• [SkipLocalsInit] attribute: By default, the CLR zero-initializes all local variables. [SkipLocalsInit] (applied to a method or module) skips this, saving cycles when you have many large stack-allocated buffers. Only safe when you guarantee initialization before first read. Apply at method level, not module level, unless you audit every method.

• Aggressive inlining: The JIT inlines methods below approximately 32 IL bytes automatically. For hot path methods just above this threshold, add [MethodImpl(MethodImplOptions.AggressiveInlining)]. Avoid applying this to large methods -- it increases code size and can harm instruction-cache efficiency. Verify with [DisassemblyDiagnoser] that inlining actually occurred.

• AggressiveOptimization: [MethodImpl(MethodImplOptions.AggressiveOptimization)] tells the JIT to spend more time optimizing a method (Tier 2 immediately). Use on known-hot, stable methods.

• Avoid virtual dispatch in hot loops: Interface calls and virtual method calls require an indirect jump through the MethodTable. In tight loops processing known concrete types, prefer: generics with struct constraints (the JIT monomorphizes the generic, eliminating virtual dispatch), or sealed classes (the JIT can devirtualize sealed types).

---

Step 6: Apply SIMD and Vectorization

SIMD (Single Instruction, Multiple Data) processes multiple data elements per CPU instruction -- 4x to 32x throughput for bulk numeric or byte operations.

• Check hardware support first:

  if (Vector.IsHardwareAccelerated)
      ProcessVectorized(data);
  else
      ProcessScalar(data);
  

• System.Numerics.Vector<T> is the portable SIMD API. Vector<T>.Count gives the number of elements per vector (16 for Vector<byte> on AVX2, 8 for Vector<int>). Use it for sum-of-squares, dot products, element-wise operations on arrays:

  var sum = Vector<int>.Zero;
  int vectorSize = Vector<int>.Count;
  int i = 0;
  for (; i <= data.Length - vectorSize; i += vectorSize)
      sum += new Vector<int>(data, i);
  int result = Vector.Dot(sum, Vector<int>.One);
  // handle tail elements [i..data.Length)
  

• System.Runtime.Intrinsics gives direct access to AVX2, SSE4.2, ARM NEON, etc. Only use when Vector<T> cannot express the operation (e.g., byte shuffle, population count, horizontal min/max). Always guard with Avx2.IsSupported or Sse42.IsSupported and provide a scalar fallback.

• SearchValues<T> (net8+): For searching a small fixed set of characters or bytes in a large span -- e.g., URL parsing, CSV tokenization -- SearchValues<char> compiles to vectorized scanning automatically. Faster than IndexOfAny for sets of 5+ elements.

  private static readonly SearchValues<char> s_delimiters =
      SearchValues.Create(",;\t\r\n");
  int idx = span.IndexOfAny(s_delimiters);
  

• Always handle tail elements. Vectorized loops process N - (N % vectorWidth) elements. The remaining elements must be processed with a scalar tail loop. Failing to do so is a silent correctness bug.

---

Step 7: Use Source Generators to Eliminate Reflection

Reflection-based serialization, logging, and mapping are among the most common hidden allocation sources in .NET applications.

• System.Text.Json source generation: Replace runtime reflection with compile-time generated serializers:

  [JsonSerializable(typeof(OrderDto))]
  [JsonSerializable(typeof(List<OrderDto>))]
  internal partial class AppJsonContext : JsonSerializerContext { }
  
  // Usage:
  string json = JsonSerializer.Serialize(order, AppJsonContext.Default.OrderDto);
  

  This eliminates reflection, reduces startup time, and is AOT-compatible. In hot-path HTTP handlers this can reduce serialization allocations by 60--80%.

• Microsoft.Extensions.Logging compile-time log messages: Replace _logger.LogInformation("User {UserId} logged in", userId) (which boxes the userId and allocates a string) with:

  [LoggerMessage(Level = LogLevel.Information, Message = "User {UserId} logged in")]
  private static partial void LogUserLoggedIn(ILogger logger, int userId);
  

  The generated code avoids boxing and only evaluates the message string if the log level is enabled.

• Incremental source generators for custom hot paths: Write incremental source generators (using IIncrementalGenerator) for repetitive patterns like: fast property mapping (instead of AutoMapper reflection), fast enum-to-string conversion (instead of Enum.GetName), or pre-compiled regular expressions via [GeneratedRegex]:

  [GeneratedRegex(@"^\d{4}-\d{2}-\d{2}$", RegexOptions.Compiled)]
  private static partial Regex DatePattern();
  

  [GeneratedRegex] compiles the regex at build time, eliminating the runtime compilation cost and reducing allocations during matching.

---

Step 8: Validate, Document, and Protect the Optimization

Optimizations rot without discipline. Build guardrails.

• Commit BenchmarkDotNet results to version control. Store the BenchmarkDotNet.Artifacts folder output (or a summary CSV) alongside the code. Review benchmark diffs in PR descriptions.

• Write regression tests for performance. In CI, run benchmarks with --filter * and compare against the baseline using --join mode, or write a unit test using dotnet-benchmark assert extensions that fails if throughput drops by more than 10%.

• Document WHY the optimization exists. Add an XML doc comment or a // PERF: comment block explaining: what profiling showed, what the allocation was, what the fix is, and what the measured improvement was. Example:

  // PERF: Uses stackalloc + Span<byte> instead of byte[] to avoid heap allocation.
  // Profiling (dotMemory, 2024-01-15) showed this path allocated 4 KB/request
  // at 50K RPS = 200 MB/s allocation pressure. Fix reduces to 0 B/request.
  

• Run the full test suite. Span-based optimizations frequently introduce subtle bounds bugs. Ensure unit tests cover: empty input, single-element input, exact-vector-width input, and input sizes that are not powers of two.

• Check for thread safety regressions. Pooled objects (ArrayPool, MemoryPool) introduce reuse -- verify that returned buffers are not accessed after returning them. Use dotnet-asan or Span<T> "poison" wrappers in test builds.

---

Output Format

When delivering a C# performance analysis or optimization recommendation, structure the response as follows:

## Performance Analysis: [Component Name]

### Profiling Evidence
| Metric                  | Baseline          | Target            | Tool Used        |
|-------------------------|-------------------|-------------------|------------------|
| Allocation rate          | 250 MB/s          | < 10 MB/s         | dotnet-counters  |
| Gen0 collections/sec     | 45/sec            | < 2/sec           | dotnet-counters  |
| P99 latency              | 18 ms             | < 5 ms            | dotTrace         |
| Allocated per request    | 12,400 B          | < 200 B           | BenchmarkDotNet  |
| LOH size                 | 2.1 GB (growing)  | Stable < 50 MB    | dotMemory        |

### Bottleneck Classification
- **Root cause:** [e.g., Repeated string Substring() calls in JSON tokenizer hot path]
- **Category:** Allocation pressure -- string heap churn
- **Impact scope:** Called 500,000x per second under p95 load

### Optimization Plan

| Priority | Technique                  | Expected Gain       | Complexity | Risk    |
|----------|----------------------------|---------------------|------------|---------|
| 1        | ReadOnlySpan<char> slicing | -90% allocations    | Low        | Low     |
| 2        | SearchValues<char>         | -40% CPU in scan    | Low        | Low     |
| 3        | ArrayPool<byte> for buffers| Eliminate LOH churn | Medium     | Medium  |
| 4        | Source-gen serialization   | -60% serial alloc   | Medium     | Low     |

### Implementation

#### Before (problematic pattern)
\`\`\`csharp
// PROBLEM: Substring allocates a new string on every call
public int FindField(string input, int startIndex)
{
    string slice = input.Substring(startIndex);   // heap allocation
    return int.Parse(slice.Split(',')[0]);         // two more allocations
}
\`\`\`

#### After (optimized pattern)
\`\`\`csharp
// PERF: Zero-allocation span-based parsing
// Measured: 0 B/call vs 184 B/call before (BenchmarkDotNet, net8.0)
public int FindField(ReadOnlySpan<char> input, int startIndex)
{
    ReadOnlySpan<char> slice = input[startIndex..];  // no allocation
    int comma = slice.IndexOf(',');
    ReadOnlySpan<char> field = comma >= 0 ? slice[..comma] : slice;
    return int.Parse(field, NumberStyles.Integer, CultureInfo.InvariantCulture);
}
\`\`\`

### BenchmarkDotNet Results

| Method         | Mean      | Error    | StdDev   | Gen0   | Allocated |
|----------------|-----------|----------|----------|--------|-----------|
| FindFieldOld   | 245.3 ns  | 1.42 ns  | 1.33 ns  | 0.0458 | 184 B     |
| FindFieldNew   |  38.7 ns  | 0.28 ns  | 0.26 ns  | -      | 0 B       |
| Ratio          | baseline  |          |          |        | 100%      |
|                | 6.3x faster|         |          |        | 0%        |

### Validation Checklist
- [ ] Unit tests pass for: empty span, single-char, no-comma, exact-buffer-size inputs
- [ ] BenchmarkDotNet baseline committed to repo
- [ ] PERF comment added explaining rationale and measurement date
- [ ] `[MemoryDiagnoser]` confirms 0 B allocated on hot path
- [ ] Load test confirms p99 improvement in staging environment

---

Rules

1. Never use string.Substring() in hot paths. It allocates a new string on every call. Replace with ReadOnlySpan<char> slicing (span[start..end]). If you must produce a string at the boundary, call span.ToString() exactly once.

2. Never benchmark in Debug configuration. The C# compiler and JIT disable optimizations in Debug builds. Always use dotnet run -c Release or dotnet build -c Release before benchmarking. Results from Debug builds are not only meaningless -- they actively mislead.

3. Never store a Span<T> in a field, async method, or iterator. The compiler enforces this for ref struct, but understand WHY: the Span<T> may point to stack memory that is no longer valid after the method returns. Use Memory<T> when cross-scope storage is needed.

4. Never return a rented ArrayPool<T> buffer more than once. Double-return corrupts the pool's free list and causes subsequent callers to receive the same buffer, leading to data corruption that is extremely difficult to debug. Use IMemoryOwner<T> and IDisposable to enforce single-return semantics.

5. Never apply [MethodImpl(MethodImplOptions.AggressiveInlining)] without verifying with [DisassemblyDiagnoser]. The JIT may refuse to inline even with the hint (e.g., method is too large, contains try/catch, or is a P/Invoke). The attribute does not guarantee inlining -- verify it happened.

6. Never use Enum.GetName(), Enum.ToString(), or .ToString("G") in hot paths. These use reflection and allocate. Use a [GeneratedRegex]-style source generator, a static ReadOnlySpan<char>-indexed lookup table, or the FastEnum pattern (switch expression over known values).

7. Always handle the tail when vectorizing. Vectorized loops must follow with a scalar tail loop for data.Length % Vector<T>.Count remaining elements. Omitting the tail loop is a silent correctness bug that only manifests on inputs not divisible by the vector width.

---

Edge Cases

Large Object Heap Fragmentation

Symptom: loh-size growing indefinitely in dotMemory, Gen2 GC frequency spiking, but no object leaks visible. Cause: byte[] or string allocations >= 85,000 bytes (the LOH threshold) that have long-enough lifetimes to survive at least one GC, fragmenting the LOH address space. Fix: Pool all large buffers with ArrayPool<byte>.Shared -- rented arrays stay in pool memory which is already on the LOH and is reused. For strings, avoid large concatenations; use StringBuilder with a pooled backing store. If you need LOH compaction once, call GCSettings.LargeObjectHeapCompactionMode = GCLargeObjectHeapCompactionMode.CompactOnce before GC.Collect(2, GCCollectionMode.Forced) in a maintenance window -- do not do this on a hot path.

ref struct Incompatibility with Existing APIs

A method returning Span<T> or accepting Span<T> cannot implement an interface method, be used as a generic type argument in non-ref struct generics, or be stored in a class field. If you need to pass Span<T>-based processing results to a component that only accepts IEnumerable<T> or T[]: create a seam -- have the Span<T> processing layer call a callback/delegate with the result rather than returning it, or materialize to a pooled array with explicit lifetime control using IMemoryOwner<T>. Do not fight the type system by casting -- it indicates an architectural boundary issue.

Benchmarking Multi-Core / Concurrent Paths

BenchmarkDotNet defaults to single-threaded execution. If the optimized code is accessed concurrently (e.g., a shared cache, a ConcurrentQueue<T>, a channel reader), single-threaded benchmarks will not reveal cache-line contention or lock convoy effects. Use [ThreadingDiagnoser] and [Benchmark] with OperationsPerInvoke set to a batch size, and run the benchmark from multiple threads by using Parallel.For inside the benchmark method. Alternatively, use load testing tools (k6, wrk2) against a local server instance for concurrency-sensitive paths.

Source Generator Conflicts and Incremental Build Issues

When multiple source generators run in the same compilation, they can produce conflicting partial class members or duplicate type names. Symptoms: CS0101 The namespace already contains a definition or CS0111 Member is already defined with same parameter types. Fix: ensure each generator emits into distinct partial methods with unique names, use hintName uniqueness in SourceProductionContext.AddSource(), and add #nullable enable at the top of generated files. Incremental generators must use value equality on their SyntaxValueProvider to avoid full re-generation on every keystroke -- verify with the IncrementalGeneratorInitializationContext RegisterSourceOutput vs RegisterImplementationSourceOutput distinction.

.NET Framework vs .NET 8+ API Availability

Span<T> and Memory<T> exist in netstandard2.1 but NOT in netstandard2.0 or .NET Framework 4.x. SearchValues<T> is net8+ only. [GeneratedRegex] is net7+. [LoggerMessage] source generation is net6+. If targeting netstandard2.0 for library compatibility: use Microsoft.Bcl.Memory NuGet package for Span<T> and Memory<T> backport. Use conditional compilation (#if NET8_0_OR_GREATER) to enable advanced paths on modern runtimes while providing a correct (slower) fallback on older targets. Never use #if to silently skip correctness -- only to swap implementations.

SIMD Producing Incorrect Results on Different Hardware

Vector<T>.Count varies by CPU: 4 elements for Vector<int> on SSE2, 8 on AVX2, 16 on AVX-512. Code that hardcodes a vector width (e.g., new Vector<int>(data, i) when data.Length was sized for Count == 8 only) will produce incorrect results on machines with different SIMD widths. Always derive loop bounds dynamically from Vector<int>.Count, and always write and test the scalar tail path. Run BenchmarkDotNet on at least two machines with different SIMD capability levels before shipping vectorized code to production.

GC Interaction with fixed Statements and GCHandle

When pinning managed memory with fixed or GCHandle.Alloc(obj, GCHandleType.Pinned) for P/Invoke or unsafe operations, pinned objects block GC heap compaction. If many objects are pinned simultaneously (e.g., in a tight loop with native interop), heap fragmentation increases rapidly. Fix: pin for the minimum possible duration -- enter fixed, do the native call, exit fixed immediately. For long-duration pinning (e.g., a buffer passed to an async I/O operation), prefer NativeMemory.Alloc() (net6+) or MemoryPool<byte> backed by pinned managed memory to minimize impact on the GC heap.

Struct Defensive Copies from readonly Violations

The most insidious struct performance bug: a struct field on a readonly context (a readonly field, an in parameter, a foreach iteration variable) that has a non-readonly method called on it causes the JIT to silently emit a defensive copy of the entire struct before each call. Symptoms: profiling shows unexpected memory writes in what should be a read-only path; performance scales with struct size. Diagnosis: enable Roslyn analyzer IDE0064 or use [DisassemblyDiagnoser] and look for unexpected lea/mov sequences. Fix: declare the struct readonly, or mark methods that do not mutate state as readonly methods (public readonly int ComputeHash() { ... }).

---

Example

User input: "I have an ASP.NET Core API endpoint that parses incoming CSV-like log lines and extracts three fields. Under load (20K RPS), dotnet-counters shows 180 MB/s allocation rate and Gen0 GC running 80 times per second. The method is called on every request. How do I optimize it?"

---

Performance Analysis: Log Line Parser Hot Path

Profiling Evidence

Metric	Baseline	Target	Tool Used
Allocation rate	180 MB/s	< 5 MB/s	dotnet-counters
Gen0 collections/sec	80/sec	< 5/sec	dotnet-counters
Allocated per request	~9,000 B	< 64 B	BenchmarkDotNet
P95 latency	22 ms	< 4 ms	dotTrace
Throughput	14K RPS (degraded)	20K+ RPS	load test

Bottleneck Classification

• Root cause: string.Split() called on every log line allocates a string[] and N new string objects per call. With 20K RPS and average 6 fields per line, this generates ~120,000 string allocations per second.
• Category: Allocation pressure -- string heap churn causing Gen0 GC storm
• Impact scope: Called synchronously on every request in the middleware pipeline

Original Problematic Code

// PROBLEM: Three allocations per call minimum: Split array + two Substring results
// Under 20K RPS = ~240,000 allocations/sec, ~9 KB/request
public static (string Timestamp, string Level, string Message) ParseLogLine(string line)
{
    var parts = line.Split('|');          // allocates string[] + N string objects
    return (
        parts[0].Trim(),                  // allocates trimmed string
        parts[1].Trim(),                  // allocates trimmed string
        parts[2]                          // references existing string (ok)
    );
}

Why This Is Expensive

Every string.Split('|') allocates:

1. A string[] of length N (24 + N * 8 bytes on 64-bit)
2. N new string objects, one per segment
3. Each Trim() call potentially allocates another string if whitespace is present

At 20K RPS with average 5-field lines: approximately 160,000 string objects per second hitting the GC.

Optimized Implementation

// PERF: Zero-allocation log line parser using ReadOnlySpan<char>
// Measured: 0 B/call vs ~9,200 B/call before (BenchmarkDotNet, net8.0, 2024-01-20)
// dotnet-counters alloc-rate: 180 MB/s -> 1.2 MB/s at 20K RPS

public ref struct LogFields
{
    public ReadOnlySpan<char> Timestamp;
    public ReadOnlySpan<char> Level;
    public ReadOnlySpan<char> Message;
}

public static bool TryParseLogLine(
    ReadOnlySpan<char> line,
    out LogFields fields)
{
    fields = default;

    int first = line.IndexOf('|');
    if (first < 0) return false;

    int second = line[(first + 1)..].IndexOf('|');
    if (second < 0) return false;
    second += first + 1; // adjust to absolute index

    fields.Timestamp = line[..first].Trim();
    fields.Level      = line[(first + 1)..second].Trim();
    fields.Message    = line[(second + 1)..];
    return true;
}

// Caller: accept ReadOnlySpan<char> from request body or Pipe reader
// to avoid ever materializing a string if downstream processing can work with spans.
// If a string IS needed for logging/storage, call span.ToString() exactly once at boundary.

ASP.NET Core Integration Pattern

// In the controller or minimal API handler:
// Use PipeReader to read body as ReadOnlySequence<byte>, then process without string allocation
app.MapPost("/ingest", async (HttpContext ctx) =>
{
    var reader = ctx.Request.BodyReader;
    ReadResult result = await reader.ReadAsync();
    ReadOnlySequence<byte> buffer = result.Buffer;

    // Process each line without allocating strings
    foreach (var segment in buffer)
    {
        // MemoryMarshal.Cast to char if UTF-8 encoding is guaranteed ASCII subset
        // Otherwise use a pooled decoder:
        Span<char> charBuffer = stackalloc char[512]; // safe: lines are bounded
        int charCount = Encoding.UTF8.GetChars(segment.Span, charBuffer);
        ReadOnlySpan<char> line = charBuffer[..charCount];

        if (TryParseLogLine(line, out LogFields fields))
        {
            // Process fields -- all are spans, zero allocations
            ProcessFields(fields.Timestamp, fields.Level, fields.Message);
        }
    }

    reader.AdvanceTo(buffer.End);
    return Results.Ok();
});

BenchmarkDotNet Setup and Results

[MemoryDiagnoser]
[SimpleJob(RuntimeMoniker.Net80)]
public class LogParserBenchmarks
{
    private readonly string _line = "2024-01-20T10:30:00Z|INFO|User authenticated successfully";

    [Benchmark(Baseline = true)]
    public (string, string, string) ParseWithSplit()
    {
        var parts = _line.Split('|');
        return (parts[0].Trim(), parts[1].Trim(), parts[2]);
    }

    [Benchmark]
    public bool ParseWithSpan()
    {
        return TryParseLogLine(_line.AsSpan(), out _);
    }
}

Method	Mean	Error	StdDev	Ratio	Gen0	Allocated	Alloc Ratio
ParseWithSplit	187.4 ns	1.12 ns	1.05 ns	1.00	0.0286	120 B	1.00
ParseWithSpan	31.2 ns	0.18 ns	0.17 ns	0.17	--	0 B	0.00

Result: 6x faster, 0 bytes allocated per call vs 120 bytes.

At 20K RPS: allocation rate drops from ~2.4 GB/min to 0 MB/min for this path.

Source Generator Addition for Structured Logging

Replace the log write that would re-allocate strings for the logger:

// Before: boxes level string, allocates interpolated string
_logger.LogInformation("Parsed log: timestamp={Timestamp} level={Level}", ts, level);

// After: compile-time generated, no boxing, no allocation when log level disabled
[LoggerMessage(Level = LogLevel.Information,
    Message = "Parsed log: timestamp={Timestamp} level={Level}")]
private static partial void LogParsedEntry(
    ILogger logger,
    ReadOnlySpan<char> timestamp,  // net8+: LoggerMessage supports Span params
    ReadOnlySpan<char> level);

Validation Checklist

• Unit tests cover: empty line, single-field line, no second delimiter, leading/trailing whitespace in fields, Unicode characters in Message field, exactly 512-char line (stack boundary)
• BenchmarkDotNet results committed to benchmarks/LogParser_net8_results.json
• [MemoryDiagnoser] confirms 0 B allocated on optimized path
• Load test at 20K RPS in staging: Gen0 GC rate dropped from 80/sec to 3/sec
• PERF: comment block added to TryParseLogLine with profiling date and before/after numbers
• dotnet-counters monitoring confirmed alloc-rate dropped from 180 MB/s to 1.2 MB/s (residual from unrelated paths)
• No regressions in integration test suite (dotnet test -c Release)