logfmt.net

Fast, structured logging for .NET in logfmt format

$ dotnet add package logfmt.net click to copy

ts=2026-03-22T03:44:07Z level=info msg="User logged in" user_id=123 service=api

GitHub NuGet

⚡

High Performance

Thread-static StringBuilder reuse, cached severity strings, zero-alloc filtered calls. Built for hot paths.

🔌

Microsoft.Extensions.Logging

Drop-in ILoggerProvider. One line to wire up with ASP.NET Core, Generic Host, or Minimal APIs.

📡

OpenTelemetry

Export OpenTelemetry log records in logfmt format with trace context, attributes, and exceptions.

🎯

Modern .NET

Targets .NET 8.0 and .NET 10.0. Nullable reference types, file-scoped namespaces, implicit usings.

🔧

Flexible Output

Console by default, or any Stream. WithData() builder pattern for default fields across log entries.

🛡️

Thread Safe

Shared write locks across WithData-derived loggers. Safe for concurrent multi-threaded logging.

Get started in seconds

Basic Usage

using Logfmt;

var log = new Logger();
log.Info("Server started", "port", "8080");

// Typed values — int, bool, double, etc.
log.Info("Request handled", "status", 200, "ms", 42);

// Default fields on every entry
var svc = new Logger().WithData("service", "api");
svc.Info("Ready");

ASP.NET Core

using Logfmt.ExtensionLogging;

builder.Logging.ClearProviders();
builder.Logging.AddLogfmt(config =>
{
    config.LogLevel["Default"] = LogLevel.Information;
});

// Then inject and use ILogger as usual
_logger.LogInformation("Processing {RequestId}", id);

OpenTelemetry

using Logfmt.OpenTelemetryLogging;

builder.Logging.AddOpenTelemetry(options =>
{
    options.AddLogfmtConsoleExporter();
});

Parsing

using Logfmt;

var pairs = LogfmtParser.Parse(
    "level=info msg=\"hello\" user_id=123");

// pairs[0] = { "level", "info" }
// pairs[1] = { "msg", "hello" }
// pairs[2] = { "user_id", "123" }