Overview

Availability: .NET 6

Documentation: https://learn.microsoft.com/en-us/dotnet/core/extensions/logger-message-generator

The LoggerMessageAttribute, part of Microsoft.Extensions.Logging, source-generates performant logging APIs. The auto-generated code relies on ILogger and LoggerMessage.Define.

This implementation is significantly faster than other approaches).

Usage

Use LoggerMessageAttribute on partial logging methods:

public static partial class Log
{
    [LoggerMessage(
        EventId = 0,
        Level = LogLevel.Critical,
        Message = "Could not open socket to `{hostName}`")]
    public static partial void CouldNotOpenSocket(ILogger logger, string hostName);
}

The method does not have to be static. Example on an instance method:

public partial class InstanceLoggingExample
{
    private readonly ILogger _logger;

    public InstanceLoggingExample(ILogger logger)
    {
        _logger = logger;
    }

    [LoggerMessage(
        EventId = 0,
        Level = LogLevel.Critical,
        Message = "Could not open socket to `{hostName}`")]
    public partial void CouldNotOpenSocket(string hostName);
}

The log level can also be dynamically set:

public static partial class Log
{
    [LoggerMessage(
        EventId = 0,
        Message = "Could not open socket to `{hostName}`")]
    public static partial void CouldNotOpenSocket(
        ILogger logger,
        LogLevel level, /* Dynamic log level as parameter, rather than defined in attribute. */
        string hostName);
}

Logging Message Constraints

Logging methods must adhere to these constraints:

They must be partial and return void
Their names cannot start with an underscore
Their parameter names cannot start with an underscore
They cannot be defined in a nested type
They cannot be generic