SyntaxCircus.AspNetCore.Common 0.1.5

SyntaxCircus.AspNetCore.Common

The small pieces of ASP.NET Core host boilerplate that show up in nearly every project, in one place: correlation-ID middleware, security headers, a composable exception-handler/HSTS bootstrap, RFC 7807 ProblemDetails exception handling, trusted-proxy validation, standard health check endpoints, and rate-limiting policy helpers.

No support guaranteed. Published as-is and maintained on a best-effort basis. Issues and PRs are welcome, but there's no SLA — fork it or vendor what you need if that's not enough.

Correlation ID

builder.Services.AddCorrelationId(); // optionally: options => options.HeaderName = "X-My-Correlation-Id"

var app = builder.Build();
app.UseCorrelationId();

Reads (or generates) a correlation ID per request, echoes it on the response header, tags the current Activity, and pushes it — with the current trace/span ID — into the logger scope for the rest of the request, so downstream log lines carry it automatically. SyntaxCircus.AspNetCore.Common.CorrelationContextAccessor.CurrentCorrelationId gives ambient (AsyncLocal) access to it outside the middleware pipeline.

Security headers

builder.Services.AddSecurityHeaders(builder.Configuration); // binds the "SecurityHeaders" section

var app = builder.Build();
app.UseSecurityHeaders();

Sets Referrer-Policy, X-Frame-Options, X-Content-Type-Options, Permissions-Policy, Content-Security-Policy, and Strict-Transport-Security from SecurityHeadersOptions, with sensible defaults you can override per-key in configuration. X-Robots-Tag is also supported (RobotsTag) but omitted entirely unless set.

Some routes need different values than the rest of the app — e.g. a public, unauthenticated page that must never leak a referrer or get indexed. Configure PathOverrides for that instead of writing a second middleware:

{
  "SecurityHeaders": {
    "PathOverrides": [
      { "PathPrefix": "/public-profile", "ReferrerPolicy": "no-referrer", "RobotsTag": "noindex, nofollow" }
    ]
  }
}

The first entry whose PathPrefix matches the request path (via PathString.StartsWithSegments) wins; everything else keeps the top-level defaults.

Exception handling / HSTS bootstrap

var app = builder.Build();
app.UseStandardExceptionHandling(); // "/error", HSTS — skipped entirely in Development

Bundles UseExceptionHandler(errorPath) + UseHsts(), both skipped in Development, with an optional status-code re-execute page (useStatusCodePages: true). It's a plain extension method, not something wired in automatically — a pure API host behind a reverse proxy that already terminates TLS and handles error pages doesn't need to call it.

ProblemDetails exception handling

builder.Services.AddProblemDetailsExceptionHandling(options =>
{
    options.BaseTypeUri = "https://errors.example.com";
    options.ExceptionMapper = ex => ex switch
    {
        NotFoundException => new ProblemMapping(StatusCodes.Status404NotFound, "not-found"),
        _ => new ProblemMapping(StatusCodes.Status500InternalServerError, "internal-error"),
    };
});

var app = builder.Build();
app.UseProblemDetailsExceptionHandling();

Catches unhandled exceptions and writes an RFC 7807 ProblemDetails response, with Type built from BaseTypeUri + your error code. Which exception types mean what status/code is deliberately a delegate you supply — that mapping is product-specific and the package doesn't try to guess it for you. A reasonable default mapper is provided if you don't set one, and it never puts a raw ex.Message into the response body: every case — including the unmapped/500 fallback — gets an explicit, generic Detail string. This matters because ex.Message on an exception nobody anticipated (a database error, a file path, connection details) can carry internals that shouldn't reach an API client.

The same rule applies to custom mappers: if your ExceptionMapper leaves a ProblemMapping's Detail unset (null), the middleware leaves Detail null in the response too — it does not silently substitute ex.Message. If you want the old fall-back-to-ex.Message behavior for cases your mapper doesn't set Detail for, opt in explicitly:

builder.Services.AddProblemDetailsExceptionHandling(options =>
{
    options.IncludeExceptionMessageInDetail = true; // restores ex.Message fallback when a mapping's Detail is null
});

IncludeExceptionMessageInDetail defaults to false. Only set it to true if you've verified your exception messages are safe to expose to API clients (e.g. gated to non-production environments).

Trusted-proxy validation

builder.Services.AddTrustedProxyForwardedHeaders(builder.Configuration); // binds the "TrustedProxy" section

var app = builder.Build();
app.UseForwardedHeaders();

Wires ForwardedHeadersOptions (X-Forwarded-For/-Proto/-Host) from TrustedProxies/TrustedNetworks, and automatically fails fast at startup if you're running behind a reverse proxy without telling ASP.NET Core which upstream hosts to actually trust — with neither configured, forwarded headers would otherwise be trusted from anyone. This validation runs on its own (via a registered IStartupFilter); you don't need to call anything else for it to take effect.

If you want to trigger the same check outside the normal startup path (e.g. in a test), ValidateTrustedProxyConfiguration is available directly:

builder.Environment.ValidateTrustedProxyConfiguration(trustedProxyOptions); // throws outside Development if misconfigured

Health checks

var app = builder.Build();
app.MapStandardHealthChecks(); // /health/live (no checks run) and /health/ready (checks tagged "ready")

Maps standard liveness/readiness endpoints rendered via HealthCheckResponseWriter (status, total duration, and per-check name/status/duration/description as JSON). Register your own IHealthCheck implementations with AddHealthChecks().AddCheck<T>(tags: ["ready"]) as usual — this just standardizes the endpoints and response shape.

Pass metadataFactory to include extra data (e.g. an app version) under a metadata key — invoked per-request, omitted entirely when not supplied:

app.MapStandardHealthChecks(metadataFactory: _ => new Dictionary<string, object?> { ["version"] = appVersion });

Rate limiting

builder.Services.AddRateLimiter(options =>
{
    options.AddPerIpFixedWindow("public", permitLimit: 60, window: TimeSpan.FromMinutes(1));
    options.AddPerSubjectFixedWindow("authenticated", permitLimit: 600, window: TimeSpan.FromMinutes(1));
    options.UseProblemDetailsRejection();
});

var app = builder.Build();
app.UseRateLimiter();

API surface

Advanced partition-key composition (additive, non-breaking)

builder.Services.AddRateLimiter(options =>
{
    options.AddPartitionedFixedWindow(
        "tenant-route",
        ctx => $"{ctx.User.FindFirst("tenant_id")?.Value ?? "anon"}:{ctx.Request.Path}",
        permitLimit: 120,
        window: TimeSpan.FromMinutes(1),
        configure: fixedWindow =>
        {
            fixedWindow.QueueLimit = 5;
            fixedWindow.QueueProcessingOrder = QueueProcessingOrder.OldestFirst;
        });

    options.UseProblemDetailsRejection();
});

This keeps the existing convenience helpers intact while adding custom partition selection and optional per-policy overrides when app-level policy composition needs to be richer.

Chained multi-tier global limiter (additive, non-breaking)

For defense-in-depth rate limiting — e.g. requiring a request to pass both a per-actor and a per-IP quota, so a leaked token can't bypass IP-level throttling and vice versa — compose independently-partitioned tiers into RateLimiterOptions.GlobalLimiter:

builder.Services.AddRateLimiter(options =>
{
    options.UseChainedGlobalLimiter(
        RateLimiterOptionsExtensions.CreateFixedWindowTier(
            ctx => ctx.User.FindFirst("sub")?.Value,
            permitLimit: 600,
            window: TimeSpan.FromMinutes(1),
            isExempt: ctx => ctx.Request.Path.StartsWithSegments("/health")),
        RateLimiterOptionsExtensions.CreateFixedWindowTier(
            ctx => ctx.Connection.RemoteIpAddress?.ToString(),
            permitLimit: 60,
            window: TimeSpan.FromMinutes(1),
            isExempt: ctx => ctx.Request.Path.StartsWithSegments("/health")));

    options.UseProblemDetailsRejection();
});

A request must pass every tier's limiter to proceed; isExempt bypasses an individual tier (e.g. for health-check paths) without disabling the others.

Contributing

Issues and pull requests are welcome:

• Keep changes focused, with a clear description of the behavior change.
• Match the existing code style (see .editorconfig).
• Call out any breaking changes to the public API in your PR description.

License

MIT — see LICENSE.txt.