csharp-logger-message

name

csharp-logger-message

description

Use the [LoggerMessage] source generator for ALL logging in .NET 6+. Produces zero-allocation, compile-time-safe log methods. Trigger whenever the user writes or reviews any logging code in C#, mentions ILogger, log levels, log messages, or structured logging, even if they don't ask for LoggerMessage by name. Always use this skill when adding, updating, or reviewing logging in .NET projects — never suggest `_logger.LogInformation()` or similar extension methods directly.

LoggerMessage Source Generator Skill

Use the [LoggerMessage] attribute on partial methods to generate high-performance structured logging code at compile time. This eliminates boxing, runtime template parsing, and unnecessary allocations.

Why this matters

_logger.LogInformation("Hello {Name}", name) is convenient but pays a runtime cost: value-type boxing, string-template parsing on every call, and extra allocations. The source generator moves all of that work to compile time, producing code equivalent to hand-written LoggerMessage.Define delegates — but safer and with less boilerplate.

Analyzer rule CA1848 enforces this pattern.

Quick reference

Concept	See
Attribute properties, constraints, basic usage	basics.md
File/class organisation patterns	patterns.md
Exceptions, dynamic log levels, format specifiers, redaction	advanced.md
Common mistakes and how to avoid them	pitfalls.md

Essential rules at a glance

• The declaring class and method must be partial.
• Return type must be void.
• ILogger is required — either as a parameter (static method) or as a field/primary constructor param (instance method).
• Use PascalCase for template placeholders: {BookId}, {UserId}.
• Exception must be the first positional parameter after ILogger (it is treated specially by the runtime).
• Do not use string interpolation in Message — placeholders only.

Minimal example

public static partial class Log
{
    [LoggerMessage(
        EventId = 1001,
        Level = LogLevel.Information,
        Message = "Book {BookId} created by {UserId}")]
    public static partial void BookCreated(ILogger logger, Guid bookId, Guid userId);
}

Calling site:

Log.BookCreated(_logger, book.Id, userId);

Organisation strategy

For any non-trivial service, split log messages into a central partial class split across feature-area files. See patterns.md for the recommended layout.

What to read next

• New to the pattern → basics.md
• Adding Exceptions, dynamic levels, or sensitive-data redaction → advanced.md
• Getting compiler errors or unexpected behaviour → pitfalls.md