Logging in ASP.NET Core — ILogger, Serilog, NLog

Introduction

When ShopNest checkout fails at 2 AM, logs tell you why. ASP.NET Core ships with ILogger<T>; production apps add Serilog for structured logs to files, Seq, or Application Insights.

After this article you will

• Use log levels appropriately
• Write structured logs with message templates
• Configure Serilog sinks and enrichers
• Add correlation IDs per request
• Mask sensitive data in log output

Prerequisites

• Article 26 — Filters
• ShopNest.Web with EF Core from Module 2

Concept deep-dive

// Structured — DO NOT string interpolate for production analytics
_logger.LogInformation("Order {OrderId} paid {Amount} by {UserId}",
    order.Id, order.Total, userId);

// Serilog Program.cs
Log.Logger = new LoggerConfiguration()
    .ReadFrom.Configuration(builder.Configuration)
    .Enrich.FromLogContext()
    .Enrich.WithProperty("App", "ShopNest")
    .WriteTo.Console()
    .WriteTo.File("logs/shopnest-.txt", rollingInterval: RollingInterval.Day)
    .CreateLogger();
builder.Host.UseSerilog();

Correlation ID: middleware sets TraceIdentifier or custom header — flows through all logs for one request.

Masking: never log full card numbers, passwords, or JWT tokens.

Hands-on — ShopNest Production E-Commerce Monitoring

1. Add Serilog NuGet + appsettings Serilog section.
2. CorrelationIdMiddleware pushes ID into LogContext.
3. PaymentService logs Information on success, Error on gateway exception.
4. Filter appsettings: Microsoft.EntityFrameworkCore at Warning in Production.

Common errors & best practices

• LogInformation for every loop iteration — noise and cost.
• String interpolation in logs — loses structured properties.
• Sync file writes under load — Serilog async sink helps.

Interview questions

Q: Structured logging benefit?
A: Query logs by OrderId, UserId in Seq/App Insights — not grep plain text.

Q: Serilog vs built-in?
A: Built-in abstracts providers; Serilog is rich sink/enricher ecosystem.

Q: Correlation ID purpose?
A: Tie all log lines from one HTTP request together in distributed traces.

Summary

• ILogger with structured templates is the baseline
• Serilog sinks feed files, Seq, Application Insights
• Correlation IDs essential for production debugging
• Mask PCI/PII — never log secrets

Previous: Filters
Next: Error Handling and Exception Management

FAQ