Serilog

using Serilog;
// Stage 1: Bootstrap logger — captures startup errors before DILog.Logger = new LoggerConfiguration()    .MinimumLevel.Override("Microsoft", LogEventLevel.Information)    .Enrich.FromLogContext()    .WriteTo.Console()    .CreateBootstrapLogger();
try{    Log.Information("Starting application");
    var builder = WebApplication.CreateBuilder(args);
    // Stage 2: Full logger with DI and configuration    builder.Services.AddSerilog((services, lc) => lc        .ReadFrom.Configuration(builder.Configuration)        .ReadFrom.Services(services)        .Enrich.FromLogContext()        .Enrich.WithMachineName()        .Enrich.WithEnvironmentName()        .Enrich.WithProperty("Application", "MyApp.Api"));
    var app = builder.Build();
    app.UseSerilogRequestLogging(options =>    {        options.EnrichDiagnosticContext = (diagnosticContext, httpContext) =>        {            diagnosticContext.Set("RequestHost", httpContext.Request.Host.Value);            diagnosticContext.Set("UserAgent",                httpContext.Request.Headers.UserAgent.ToString());        };    });
    app.Run();}catch (Exception ex){    Log.Fatal(ex, "Application terminated unexpectedly");}finally{    await Log.CloseAndFlushAsync();}

app.UseSerilogRequestLogging(options =>{    options.MessageTemplate =        "HTTP {RequestMethod} {RequestPath} responded {StatusCode} in {Elapsed:0.0000} ms";
    options.GetLevel = (httpContext, elapsed, ex) => ex is not null        ? LogEventLevel.Error        : httpContext.Response.StatusCode >= 500            ? LogEventLevel.Error            : LogEventLevel.Information;
    options.EnrichDiagnosticContext = (diagnosticContext, httpContext) =>    {        diagnosticContext.Set("UserId",            httpContext.User.FindFirstValue(ClaimTypes.NameIdentifier) ?? "anonymous");    };});

// Named properties — creates queryable structured datalogger.LogInformation("Order {OrderId} placed by {CustomerId} for {Total:C}",    orderId, customerId, total);
// @ operator preserves object structure as propertieslogger.LogInformation("Processing {@SensorInput}", sensorInput);// Output: Processing {"Latitude": 25, "Longitude": 134}
// $ operator forces ToString()logger.LogInformation("Received {$Data}", new[] { 1, 2, 3 });// Output: Received "System.Int32[]"

using (LogContext.PushProperty("CorrelationId", correlationId))using (LogContext.PushProperty("TenantId", tenantId)){    logger.LogInformation("Processing order {OrderId}", orderId);    // CorrelationId and TenantId attached to ALL log events in this scope}

.WriteTo.OpenTelemetry(options =>{    options.Endpoint = "http://localhost:4317";    options.Protocol = OtlpProtocol.Grpc;    options.ResourceAttributes = new Dictionary<string, object>    {        ["service.name"] = "MyApp.Api",        ["deployment.environment"] = "production"    };})

// Exclude health check noise.Filter.ByExcluding("RequestPath like '/health%'")
// Route errors to a separate file.WriteTo.Conditional("@l = 'Error'",    wt => wt.File("logs/errors-.log", rollingInterval: RollingInterval.Day))

public static partial class OrderLogs{    [LoggerMessage(Level = LogLevel.Information,        Message = "Order {OrderId} created for {CustomerId}")]    public static partial void OrderCreated(this ILogger logger, Guid orderId, Guid customerId);}
// Usagelogger.OrderCreated(order.Id, order.CustomerId);

// BAD — breaks structured logging, allocates even when level is disabledlogger.LogInformation($"Order {orderId} created for {customerId}");
// GOOD — message template with named parameterslogger.LogInformation("Order {OrderId} created for {CustomerId}", orderId, customerId);

// BAD — async sinks (Seq, OTLP, Elasticsearch) lose buffered eventsapp.Run();
// GOOD — wrap in try/finallytry { app.Run(); }catch (Exception ex) { Log.Fatal(ex, "Unhandled exception"); }finally { await Log.CloseAndFlushAsync(); }

// BAD — passwords and tokens in logslogger.LogInformation("Login: {Email} with password {Password}", email, password);
// GOOD — never log secrets, passwords, tokens, or PIIlogger.LogInformation("Login: {Email}", email);

// BAD — large object graphs cause memory issues and massive log entrieslogger.LogInformation("Request: {@Request}", httpContext.Request);
// GOOD — configure destructuring limits.Destructure.ToMaximumDepth(4).Destructure.ToMaximumStringLength(1024).Destructure.ToMaximumCollectionCount(10)
// BETTER — destructure to specific properties.Destructure.ByTransforming<HttpRequest>(r => new { r.Method, r.Path })

// BAD — the Serilog.Sinks.Elasticsearch PACKAGE is deprecated// <PackageReference Include="Serilog.Sinks.Elasticsearch" />.WriteTo.Elasticsearch("http://localhost:9200")
// GOOD — same method name, but from the official Elastic.Serilog.Sinks// package, which writes ECS-formatted documents to data streams// <PackageReference Include="Elastic.Serilog.Sinks" />.WriteTo.Elasticsearch([new Uri("https://elastic.example.com:9200")], opts =>    opts.DataStream = new DataStreamName("logs", "myapp"))