serilog

Serilog

Core Principles

1. Two-stage initialization — Create a bootstrap logger for startup, then replace it with the full logger after DI is ready. This captures startup errors that would otherwise be lost.
2. AddSerilog() over UseSerilog() — Use builder.Services.AddSerilog() (the modern API) instead of builder.Host.UseSerilog(). It integrates with DI services via ReadFrom.Services(services).
3. Message templates, not interpolation — {PropertyName} syntax creates structured data that can be queried. String interpolation ($"...") breaks structure and allocates even when the log level is disabled.
4. Configure via appsettings.json — Keep log levels, sinks, and overrides in configuration so they can change per environment without redeployment.

Patterns

Two-Stage Bootstrap Setup

using Serilog;

// Stage 1: Bootstrap logger — captures startup errors before DI
Log.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();
}