System Net Review

Writing & Reviewing System.Net Code

This skill provides implementation guidance when writing or modifying code under src/libraries/System.Net.*, src/libraries/System.Private.Uri, and networking-related files in src/libraries/Common/src. It also serves as a review trigger for System.Net changes.

Scope

Applies to all System.Net.* libraries, System.Private.Uri, and shared networking code in Common/src. For general dotnet/runtime conventions, defer to the path-scoped files under .github/instructions/ (style, tests), the build-and-test skill (build and test workflow), and the code-review skill.

Review Delegation

When reviewing a PR or code change that touches System.Net code, invoke the @system-net-review agent as a sub-task. The agent carries the full structured checklist with severity-weighted routing and per-folder hotspot coverage. This skill provides the decision frameworks and coding patterns; the agent provides the line-by-line review checklist.

When invoking from an existing reviewer agent, tell the @system-net-review to only collect feedback items and return them, do not post them to GH. The parent reviewer is responsible for collection, deduplication, and posting.

Do not duplicate the agent's CHECK items here. Use this skill for authoring guidance and the agent for review verdicts.

Decision Frameworks

1. Resource Lifecycle

New resource introduced?
├─ Native handle (socket, TLS context, msquic obj) → SafeHandle subclass; Release in ReleaseHandle()
├─ Managed IDisposable (Stream, HttpConnection) → single owner; dispose in finally/using
├─ Pooled resource (HttpConnectionPool entry) → return-to-pool on success; dispose on fault
└─ Temporary buffer → ArrayPool<byte>.Shared; return in finally block

Key rules:

• Every resource must have exactly one owner responsible for disposal.
• Dispose() must be idempotent and never throw.
• Error paths must dispose; use try/finally, not just using on the happy path.

2. Connection Pooling

Modifying connection pool logic?
├─ Pool sizing / lifetime → respect HttpConnectionPoolManager limits; honor PooledConnectionIdleTimeout
├─ Connection acquisition → async wait with CancellationToken; fail fast on disposed pool
├─ Connection return → validate state before reuse; drain if protocol error
└─ Connection eviction → close gracefully; log via NetEventSource

Key rules:

• Never hold a lock while performing I/O or awaiting.
• Connections returned to pool must be in a reusable state (no partial reads, no pending writes).
• Pool disposal must drain all waiters and close all connections.
• Prefer Interlocked operations over lock for counters on hot paths.

3. Cross-Platform Interop

Adding or modifying P/Invoke?
├─ New declaration → use [LibraryImport], not [DllImport]
├─ Struct layout → verify with StructLayout on all target architectures (x86, x64, arm64)
├─ String marshaling → use StringMarshalling.Utf8 for Unix, Utf16 for Windows
├─ Return values → check errno/GetLastPInvokeError(); map to SocketException/Win32Exception
└─ Lifetime → SafeHandle for any native handle; never raw IntPtr across async boundaries

Key rules:

• Platform-specific code belongs in .Windows.cs / .Unix.cs / .OSX.cs partial files.
• All Interop. declarations live under Common/src/Interop/<platform>/.
• Test on all platforms via ConditionalFact/ConditionalTheory with PlatformDetection.

4. Protocol Compliance

Touching HTTP/QUIC framing or headers?
├─ Header parsing → case-insensitive comparison; handle folded headers; reject invalid chars
├─ HTTP/2 frames → respect flow control windows; never exceed MAX_FRAME_SIZE; send GOAWAY cleanly
├─ HTTP/3 / QUIC → use MsQuic API via QuicConnection/QuicStream; handle 0-RTT carefully
├─ Content-Length → validate against actual bytes; fail on mismatch
└─ Transfer-Encoding: chunked → handle final chunk + trailers; zero-length chunk = end

Key rules:

• Follow the relevant RFC strictly (RFC 9110/9112 for HTTP, RFC 9000/9001 for QUIC).
• Protocol violations from the peer must produce clear exceptions, not silent corruption.
• Version-specific behavior must be gated on HttpRequestMessage.Version or negotiated ALPN.

5. Async Patterns

Writing async networking code?
├─ Return type → ValueTask<T> if synchronous completion is common (reads from buffer)
├─ ConfigureAwait → always ConfigureAwait(false) in library code
├─ Cancellation → accept CancellationToken; link with internal timeout tokens
├─ Locking → never await inside lock{}; use SemaphoreSlim for async mutual exclusion
└─ Completion source → PooledValueTaskSource or ManualResetValueTaskSourceCore for reuse

Key rules:

• Cancellation must interrupt I/O promptly, not just check at the next loop iteration.
• ValueTask must not be awaited more than once or stored.
• See the agent's async CHECK items for the full list.

6. Security Posture

Changing TLS, auth, or credential handling?
├─ TLS defaults → SslProtocols.None (OS-negotiated); never hardcode TLS 1.0/1.1
├─ Certificate validation → default RemoteCertificateValidationCallback must reject invalid certs
├─ Credentials → never log; never cache beyond session; clear from memory when done
├─ SslStream options → default to RequireEncryption; client cert negotiation must be explicit
└─ QUIC TLS → ensure certificate chain is validated via SslServerAuthenticationOptions

Key rules:

• Security settings must be secure by default; opt-out must be explicit and documented.
• Validate all TLS configuration before handshake; fail early with clear AuthenticationException.
• See the agent's security and diagnostics CHECK items for the full list.

Code Patterns

NetEventSource Tracing

// DO: guard with IsEnabled and pass 'this' for correlation
if (NetEventSource.Log.IsEnabled()) NetEventSource.Info(this, $"Connecting to {address}");

// DON'T: unconditional string formatting
NetEventSource.Info(this, $"Header: {header}");  // allocates even when tracing is off

// DON'T: log sensitive data
NetEventSource.Info(this, $"Auth token: {token}");  // NEVER

Socket Async Operations

// DO: reuse SocketAsyncEventArgs; set callback once
private readonly SocketAsyncEventArgs _recvArgs = new();

public MySocketHandler()
{
    _recvArgs.Completed += OnReceiveCompleted;
}
// DO: check synchronous completion
if (!_socket.ReceiveAsync(_recvArgs))
{
    OnReceiveCompleted(null, _recvArgs);
}

// DON'T: allocate new SocketAsyncEventArgs per operation
var args = new SocketAsyncEventArgs();  // hot-path allocation

Connection Pool Return

// DO: validate state before returning to pool
if (connection.IsUsable && !connection.HasUnreadData)
{
    pool.ReturnConnection(connection);
}
else
{
    connection.Dispose();
}

// DON'T: return without checking state
pool.ReturnConnection(connection);  // may reuse a broken connection

SslStream Configuration

// DO: let the OS negotiate the best protocol
var options = new SslClientAuthenticationOptions
{
    TargetHost = host,
    EnabledSslProtocols = SslProtocols.None, // OS default
};

// DON'T: pin to a specific version
EnabledSslProtocols = SslProtocols.Tls12,  // blocks future upgrades

Buffer Handling

// DO: rent from ArrayPool; return in finally
byte[] buffer = ArrayPool<byte>.Shared.Rent(4096);
try
{
    int bytesRead = await stream.ReadAsync(buffer.AsMemory(0, 4096), ct);
    ProcessData(buffer.AsSpan(0, bytesRead));
}
finally
{
    ArrayPool<byte>.Shared.Return(buffer);
}

// DON'T: allocate on the hot path
byte[] buffer = new byte[4096];  // GC pressure under load

HTTP/2 Flow Control

// DO: respect the peer's flow-control window before sending
int allowed = Math.Min(dataLength, _peerWindowAvailable);
await SendDataFrameAsync(streamId, data.Slice(0, allowed), ct);
_peerWindowAvailable -= allowed;

// DON'T: send data exceeding the window → protocol violation / FLOW_CONTROL_ERROR

Cancellation Plumbing

// DO: link user token with internal timeout
using var linkedCts = CancellationTokenSource.CreateLinkedTokenSource(
    cancellationToken, _connectionTimeoutCts.Token);

// DO: register cancellation to abort underlying I/O
using var reg = linkedCts.Token.UnsafeRegister(
    static s => ((Socket)s!).Dispose(), _socket);

// DON'T: ignore the token
await stream.ReadAsync(buffer);  // not cancellable

Testing Guidance

LoopbackServer Pattern

System.Net functional tests use in-process loopback servers to avoid real network dependencies. Choose the right one:

Conditional Attributes

// Platform-specific
[ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsNotWindows))]

// Feature gating
[ConditionalClass(typeof(PlatformDetection), nameof(PlatformDetection.SupportsAlpn))]

// Known failures — always cite the issue
[ActiveIssue("https://github.com/dotnet/runtime/issues/<issue-number>")]

Test Conventions

• Tests must clean up all sockets, streams, and servers — wrap in using/await using.
• Stress tests live under tests/StressTests/; don't add long-running loops to functional tests.
• General test conventions (prefer existing files, [Theory] over [Fact]) are in .github/instructions/tests.instructions.md.

Reference Links

• System.Net.Http SocketsHttpHandler source
• HTTP/2 implementation notes
• QUIC managed wrapper
• Common Interop layer
• Test infrastructure
• Build & test libraries
• Testing libraries