Logging Generation v1

Stable Telemetry SourceGenerator Reviewed 2026-09-16 purview-dev/telemetry-sourcegenerator Star on GitHub 36 activity c-sharp distributed-tracing dotnet events high-performance-logging instrumentation logging melt metrics nuget observability open-telemetry open-telemetry-csharp otel roslyn source-generator spans telemetry tracing

Logging Generation v1

Generation v1 is the previous style of log generation, built on LoggerMessage.Define from the high-performance logging libraries.

It has these limitations:

A maximum of 6 non-exception parameters.
At most one Exception parameter.
No support for expanding enumerations ([ExpandEnumerable] has no effect).
No support for [LogProperties].

Selecting v1

v1 is used automatically in LoggerGenerationMode.Auto for any method that is within the v1 limits. You can force it for all methods on an interface or assembly:

[Logger(GenerationMode = LoggerGenerationMode.V1)]
interface IOrderServiceTelemetry { }

Log generation

To generate a log entry, decorate the method with [Log] and return either void (non-scoped) or IDisposable/IDisposable? (scoped). Parameters form part of the structured log generation and are referenced by the MessageTemplate.

You can also use the semantic level attributes instead of [Log].Level:

[Trace], [Debug], [Info], [Warning], [Error], [Critical]

These support all the same properties as [Log], except Level.

Inferring

When not using multi-targeting you can omit [Log] entirely — every method on a [Logger] interface becomes a log method. Set the default level with [Logger].DefaultLevel (interface) or [LoggerGeneration].DefaultLevel (assembly).

If no level is specified on the method and an Exception parameter is present, the level is changed to Error automatically. This raises the TSG2002 diagnostic, which can safely be ignored if you are comfortable with the inferred behaviour.

Attribute reference

`[Log]`

Property	Type	Description
`Level`	`LogLevel`	The level used when defining the log entry. Available on construction. Defaults to `Information`, unless an `Exception` is detected in the parameters, then `Error`.
`MessageTemplate`	`string?`	The template used to populate the log entry. If not specified, one is generated from the prefixes and available parameters. Available on construction. Default `null`.
`EventId`	`int?`	Used when generating the `EventId`. Available on construction. Default `null` (one is generated if not supplied).
`Name`	`string?`	The name of the log entry. If not defined, the method name is used. Available on construction. Default `null`.
`GenerationMode`	`LoggerGenerationMode`	Per-method generation-mode override. `Auto` (default) inherits from the interface/assembly level.

`[Logger]`

Property	Type	Description
`DefaultLevel`	`LogLevel`	The default level when one is not provided. Available on construction. Default `Information`.
`CustomPrefix`	`string?`	Used when generating the log entry name’s prefix. When set, `PrefixType` is automatically `Custom`. Available on construction. Default `null`.
`PrefixType`	`LogPrefixType`	The prefix type used when generating the log entry name. Default `Default`.
`GenerationMode`	`LoggerGenerationMode`	Controls the generation mode for all log methods on the interface. `Auto` (default) selects the best mode per method.

`[LoggerGeneration]`

Property	Type	Description
`DefaultLevel`	`LogLevel`	The default level when one is not provided. Available on construction. Default `Information`.
`GenerationMode`	`LoggerGenerationMode`	Controls the generation mode for all log methods in the assembly. `Auto` (default) selects the best mode per method.
`DefaultPrefixType`	`LogPrefixType`	The default prefix type for all log entries in the assembly. Default `Default`.

`LogPrefixType`

Value	Description
`Default`	No prefix.
`Interface`	Uses the name of the interface.
`Class`	The name of the class used for generation (`TelemetryGenerationAttribute.ClassName` or auto-generated).
`Custom`	Used when `LoggerAttribute.CustomPrefix` is set.
`TrimmedClassName`	The interface name without the `I` prefix or `Log`, `Logger`, or `Telemetry` suffixes.

Next steps

Logging Generation v2 — the state-based mode with [ExpandEnumerable]/[LogProperties]
Logging — choosing between v1 and v2
Diagnostics — the TSG2xxx Logging rules