Roadmap › Logging & Custom Logging in .NET — ILogger, Serilog, NLog, log4net and Your Own Logger › Module: Logging Fundamentals
Lessons in this course (8)
1. What to log, log levels and structured logging
Basic★ Must knowLesson 1 of 8
| Level | Use for | Example |
|---|---|---|
| Trace | Very detailed internals; may contain sensitive data — off in production | "Entering CalculateTax with 3 lines" |
| Debug | Developer diagnostics | "Cache miss for product 42" |
| Information | Normal significant events | "Order 1001 placed by customer 42 (₹4,500)" |
| Warning | Unexpected but handled; might need attention | "Payment gateway slow (3.2 s), retrying" |
| Error | A failure of the current operation | "Failed to save order 1001" + exception |
| Critical | The app or a major part is down | "Cannot connect to database — shutting down" |
What to log: application start/stop and configuration summary, incoming requests (method, path, status, duration), important business events, external calls (target, duration, result), handled failures (warning) and unhandled exceptions (error, with stack trace), security events (logins, permission denied).
What NOT to log: passwords, tokens, full card numbers, secrets, connection strings, personal data beyond what's necessary (GDPR/DPDP), huge payloads.
Structured logging — log a message template with named properties instead of a pre-built string:
csharp
// ❌ String interpolation — one opaque string, can't query by OrderId, allocates even if the level is disabled
logger.LogInformation($"Order {order.Id} placed by {customer.Id} total {order.Total}");
// ✅ Message template — properties are stored separately and searchable
logger.LogInformation("Order {OrderId} placed by {CustomerId} total {Total}", order.Id, customer.Id, order.Total);In Seq/App Insights you can then query OrderId = 1001 or Total > 10000 across millions of entries.