High-performance logging in .NET

The LoggerMessage class exposes functionality to create cacheable delegates that require fewer object allocations and reduced computational overhead compared to logger extension methods, such as LogInformation and LogDebug. For high-performance logging scenarios, use the LoggerMessage pattern.

LoggerMessage provides the following performance advantages over Logger extension methods:

  • Logger extension methods require "boxing" (converting) value types, such as int, into object. The LoggerMessage pattern avoids boxing by using static Action fields and extension methods with strongly-typed parameters.
  • Logger extension methods must parse the message template (named format string) every time a log message is written. LoggerMessage only requires parsing a template once when the message is defined.

The sample app demonstrates LoggerMessage features with a priority queue processing worker service. The app processes work items in priority order. As these operations occur, log messages are generated using the LoggerMessage pattern.

Tip

All of the logging example source code is available in the Samples Browser for download. For more information, see Browse code samples: Logging in .NET.

Define a logger message

Use Define(LogLevel, EventId, String) to create an Action delegate for logging a message. Define overloads permit passing up to six type parameters to a named format string (template).

The string provided to the Define method is a template and not an interpolated string. Placeholders are filled in the order that the types are specified. Placeholder names in the template should be descriptive and consistent across templates. They serve as property names within structured log data. We recommend Pascal casing for placeholder names. For example, {Item}, {DateTime}.

Each log message is an Action held in a static field created by LoggerMessage.Define. For example, the sample app creates a field to describe a log message for the processing of work items:

private static readonly Action<ILogger, Exception> _failedToProcessWorkItem;

For the Action, specify:

  • The log level.
  • A unique event identifier (EventId) with the name of the static extension method.
  • The message template (named format string).

As work items are dequeued for processing the worker service app sets the:

  • Log level to LogLevel.Critical.
  • Event id to 13 with the name of the FailedToProcessWorkItem method.
  • Message template (named format string) to a string.
_failedToProcessWorkItem = LoggerMessage.Define(
    LogLevel.Critical,
    new EventId(13, nameof(FailedToProcessWorkItem)),
    "Epic failure processing item!");

Structured logging stores may use the event name when it's supplied with the event id to enrich logging. For example, Serilog uses the event name.

The Action is invoked through a strongly-typed extension method. The PriorityItemProcessed method logs a message every time a work item is being processed. Whereas, FailedToProcessWorkItem is called when (and if) an exception occurs:

protected override async Task ExecuteAsync(
    CancellationToken stoppingToken)
{
    using IDisposable? scope = _logger.ProcessingWorkScope(DateTime.Now);
    while (!stoppingToken.IsCancellationRequested)
    {
        WorkItem? nextItem = _priorityQueue.ProcessNextHighestPriority();
        try
        {
            if (nextItem is not null)
            {
                _logger.PriorityItemProcessed(nextItem);
            }
        }
        catch (Exception ex)
        {
            _logger.FailedToProcessWorkItem(ex);
        }

        await Task.Delay(1000, stoppingToken);
    }
}

Inspect the app's console output:

crit: WorkerServiceOptions.Example.Worker[13]
      Epic failure processing item!
      System.Exception: Failed to verify communications.
         at WorkerServiceOptions.Example.Worker.ExecuteAsync(CancellationToken stoppingToken) in
         ..\Worker.cs:line 27

To pass parameters to a log message, define up to six types when creating the static field. The sample app logs the work item details when processing items by defining a WorkItem type for the Action field:

private static readonly Action<ILogger, WorkItem, Exception> _processingPriorityItem;

The delegate's log message template receives its placeholder values from the types provided. The sample app defines a delegate for adding a work item where the item parameter is a WorkItem:

_processingPriorityItem = LoggerMessage.Define<WorkItem>(
    LogLevel.Information,
    new EventId(1, nameof(PriorityItemProcessed)),
    "Processing priority item: {Item}");

The static extension method for logging that a work item is being processed, PriorityItemProcessed, receives the work item argument value and passes it to the Action delegate:

public static void PriorityItemProcessed(
    this ILogger logger, WorkItem workItem) =>
    _processingPriorityItem(logger, workItem, default!);

In the worker service's ExecuteAsync method, PriorityItemProcessed is called to log the message:

protected override async Task ExecuteAsync(
    CancellationToken stoppingToken)
{
    using IDisposable? scope = _logger.ProcessingWorkScope(DateTime.Now);
    while (!stoppingToken.IsCancellationRequested)
    {
        WorkItem? nextItem = _priorityQueue.ProcessNextHighestPriority();
        try
        {
            if (nextItem is not null)
            {
                _logger.PriorityItemProcessed(nextItem);
            }
        }
        catch (Exception ex)
        {
            _logger.FailedToProcessWorkItem(ex);
        }

        await Task.Delay(1000, stoppingToken);
    }
}

Inspect the app's console output:

info: WorkerServiceOptions.Example.Worker[1]
      Processing priority item: Priority-Extreme (50db062a-9732-4418-936d-110549ad79e4): 'Verify communications'

Define logger message scope

The DefineScope(string) method creates a Func<TResult> delegate for defining a log scope. DefineScope overloads permit passing up to three type parameters to a named format string (template).

As is the case with the Define method, the string provided to the DefineScope method is a template and not an interpolated string. Placeholders are filled in the order that the types are specified. Placeholder names in the template should be descriptive and consistent across templates. They serve as property names within structured log data. We recommend Pascal casing for placeholder names. For example, {Item}, {DateTime}.

Define a log scope to apply to a series of log messages using the DefineScope method. Enable IncludeScopes in the console logger section of appsettings.json:

{
    "Logging": {
        "Console": {
            "IncludeScopes": true
        },
        "LogLevel": {
            "Default": "Information",
            "Microsoft": "Warning",
            "Microsoft.Hosting.Lifetime": "Information"
        }
    }
}

To create a log scope, add a field to hold a Func<TResult> delegate for the scope. The sample app creates a field called _processingWorkScope (Internal/LoggerExtensions.cs):

private static Func<ILogger, DateTime, IDisposable> _processingWorkScope;

Use DefineScope to create the delegate. Up to three types can be specified for use as template arguments when the delegate is invoked. The sample app uses a message template that includes the date time in which processing started:

_processingWorkScope =
    LoggerMessage.DefineScope<DateTime>(
        "Processing work, started at: {DateTime}");

Provide a static extension method for the log message. Include any type parameters for named properties that appear in the message template. The sample app takes in a DateTime for a custom time stamp to log and returns _processingWorkScope:

public static IDisposable ProcessingWorkScope(
    this ILogger logger, DateTime time) =>
    _processingWorkScope(logger, time);

The scope wraps the logging extension calls in a using block:

protected override async Task ExecuteAsync(
    CancellationToken stoppingToken)
{
    using IDisposable? scope = _logger.ProcessingWorkScope(DateTime.Now);
    while (!stoppingToken.IsCancellationRequested)
    {
        WorkItem? nextItem = _priorityQueue.ProcessNextHighestPriority();
        try
        {
            if (nextItem is not null)
            {
                _logger.PriorityItemProcessed(nextItem);
            }
        }
        catch (Exception ex)
        {
            _logger.FailedToProcessWorkItem(ex);
        }

        await Task.Delay(1000, stoppingToken);
    }
}

Inspect the log messages in the app's console output. The following result shows priority ordering of log messages with the log scope message included:

info: WorkerServiceOptions.Example.Worker[1]
      => Processing work, started at: 09/25/2020 14:30:45
      Processing priority item: Priority-Extreme (f5090ede-a337-4041-b914-f6bc0db5ae64): 'Verify communications'
info: Microsoft.Hosting.Lifetime[0]
      Application started. Press Ctrl+C to shut down.
info: Microsoft.Hosting.Lifetime[0]
      Hosting environment: Development
info: Microsoft.Hosting.Lifetime[0]
      Content root path: ..\worker-service-options
info: WorkerServiceOptions.Example.Worker[1]
      => Processing work, started at: 09/25/2020 14:30:45
      Processing priority item: Priority-High (496d440f-2007-4391-b179-09d75ab52373): 'Validate collection'
info: WorkerServiceOptions.Example.Worker[1]
      => Processing work, started at: 09/25/2020 14:30:45
      Processing priority item: Priority-Medium (dea9e3f4-d7df-46d2-b7cd-5e0232eb98a5): 'Propagate selections'
info: WorkerServiceOptions.Example.Worker[1]
      => Processing work, started at: 09/25/2020 14:30:45
      Processing priority item: Priority-Medium (089d7f0d-da72-4b55-92fe-57b147838056): 'Enter pooling [contention]'
info: WorkerServiceOptions.Example.Worker[1]
      => Processing work, started at: 09/25/2020 14:30:45
      Processing priority item: Priority-Low (6e68c4be-089f-4450-9080-1ea63fcbb686): 'Health check network'
info: WorkerServiceOptions.Example.Worker[1]
      => Processing work, started at: 09/25/2020 14:30:45
      Processing priority item: Priority-Deferred (6f324134-6bb6-455f-81d4-553ab307c421): 'Ping weather service'
info: WorkerServiceOptions.Example.Worker[1]
      => Processing work, started at: 09/25/2020 14:30:45
      Processing priority item: Priority-Deferred (37bf736c-7a26-4a2a-9e56-e89bcf3b8f35): 'Set process state'

Log level guarded optimizations

An additional performance optimization can be made by checking the LogLevel, with ILogger.IsEnabled(LogLevel) before an invocation to the corresponding Log* method. When logging isn't configured for the given LogLevel, the following statements are true:

  • ILogger.Log is not called.
  • An allocation of object[] representing the parameters is avoided.
  • Value type boxing is avoided.

For more information:

See also