Logging Generation v2

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 v2

Generation v2 is the state-based logging mode. Its output closely resembles the built-in [LoggerMessage] generator, but with additional features: dynamically generated MessageTemplates and the ability to expand array/IEnumerable parameters.

It is used automatically in LoggerGenerationMode.Auto for methods that exceed the v1 limits (more than 6 non-exception parameters, or [ExpandEnumerable]/[LogProperties] parameters).

Selecting v2

// Force v2 for the whole interface
[Logger(GenerationMode = LoggerGenerationMode.V2)]
interface IOrderServiceTelemetry { }

// Or per method
[Logger]
interface IOrderServiceTelemetry
{
    [Info(GenerationMode = LoggerGenerationMode.V2)]
    void OrderPlaced(int orderId, string customerName);
}

Log generation

As with v1, decorate a method with [Log] (or a semantic level attribute) and return either void (non-scoped) or IDisposable/IDisposable? (scoped).

The shared [Log], [Logger], [LoggerGeneration], and LogPrefixType reference tables are on the Logging Generation v1 page.

Custom message templates

[Log].MessageTemplate lets you customise the log message. Template placeholders map to method parameters:

[Logger]
interface IOrderServiceTelemetry
{
    [Info("Order {OrderId} placed for {CustomerName}")]
    void OrderPlaced(int orderId, string customerName);
}

If no template is specified, one is generated from the method name and parameters.

`[ExpandEnumerable]`

Applied to an array or IEnumerable parameter, it logs the individual elements rather than the collection as a whole.

Property	Type	Description
`MaximumValueCount`	`int`	The maximum number of elements to output. Default `5`.

[Logger]
interface IOrderServiceTelemetry
{
    [Info]
    void OrdersRetrieved(int orderId, [ExpandEnumerable(maximumValueCount: 100)] string[] orderNumbers);
}

`[LogProperties]`

The external Microsoft.Extensions.Logging.LogPropertiesAttribute (from Microsoft.Extensions.Telemetry.Abstractions) expands an object’s public properties into individual log properties.

Property	Type	Description
`OmitReferenceName`	`bool`	Whether the reference name is omitted from property names. Default `false`.
`SkipNullProperties`	`bool`	Whether null properties are skipped. Default `false`.
`Transitive`	`bool`	Whether nested objects are expanded transitively. Default `false`.

The companion LogPropertyIgnoreAttribute marks a property to be skipped during expansion.

Next steps

Logging Generation v1 — the LoggerMessage.Define mode and its limits
Logging — choosing between v1 and v2
Diagnostics — the TSG2xxx Logging rules