dotnet-source-generated-logging

name	dotnet-source-generated-logging
description	Write .NET logs with the LoggerMessage source generator for ILogger calls. USE FOR: adding or reviewing log statements built from [LoggerMessage] partial methods — message templates, log levels, event ids, exception logging, and logging scopes.
license	MIT

Source-Generated Logging

Use the LoggerMessage source generator (part of Microsoft.Extensions.Logging) to declare log statements as partial methods. Generated methods allocate nothing when the level is disabled, validate the message template against the typed parameters at compile time, and keep structured fields queryable. logger.LogInformation($"...") allocates on every call and flattens the arguments into a string.

When to use

• Adding a log call at any level (Trace, Debug, Information, Warning, Error, lifecycle).
• Reviewing code that calls logger.LogInformation($"..."), logger.Log(...), or an interpolated template.
• Attaching an exception or repeated contextual fields to log output.

Workflow

Step 1: Mark the owning type partial

Add partial to the class that declares the log methods; the generator emits their bodies.

public sealed partial class UserService(ILogger<UserService> logger)

Step 2: Declare a static partial log method per event

1. Annotate a private static partial void method with [LoggerMessage].
2. Take ILogger as the first parameter.
3. Write a template whose {Placeholder} names match the remaining typed parameters in order.
4. Choose the level in the attribute; add an event id only when the surrounding logging policy already uses ids.
5. Pass values as parameters — never interpolate or string.Format the template.

[LoggerMessage(LogLevel.Information, "User {UserId} created")]
private static partial void LogUserCreated(ILogger logger, Guid userId);

Step 3: Log exceptions through the exception parameter

Add an Exception parameter (any position after the logger). The generator binds it as the entry's exception; it does not format it into the message text.

[LoggerMessage(LogLevel.Error, "Failed to create user {UserId}")]
private static partial void LogUserCreationFailed(ILogger logger, Exception exception, Guid userId);

Step 4: Add shared context with a scope

When several entries share the same context, open a BeginScope; do not repeat the value in every template.

using var scope = logger.BeginScope(new Dictionary<string, object?>
{
    ["Email"] = user.Email,
    ["Phone"] = user.Phone
});

Step 5: Call the generated method and build

Call the method at the log site and build once — a template/parameter mismatch is a compile error.

LogUserCreated(logger, user.Id);

Validation

• The type that declares log methods is partial.
• Log statements go through [LoggerMessage] methods, not direct logger.Log* calls.
• Templates use {Placeholder} tokens matching the typed parameters — no interpolation or string.Format.
• Exceptions are passed as the Exception parameter, not concatenated into the message.
• Repeated context uses a logging scope.

Common Pitfalls

Pitfall	Correct approach
logger.LogInformation($"Created {userId}")	LogUserCreated(logger, userId) with a [LoggerMessage] method.
Interpolating into the template	Named {Placeholder} tokens bound to typed parameters.
Formatting the exception into the message string	Add an Exception parameter; the generator attaches it.
Non-static or non-partial log method	[LoggerMessage] methods must be static partial, and the type partial.
Placeholder names or count differ from parameters	Match names and order; the build fails otherwise.