Programming language: C#
License: MIT License
Tags: Logging     Logfiles     NetCore     Netstandard     File    

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Simple and efficient file logger provider for .NET Core (any version) without additional dependencies.

NuGet Release

  • very similar to standard ConsoleLogger but writes to a file
  • can append to existing file or overwrite log file on restart
  • supports a 'rolling file' behaviour and can control total log size
  • it is possible to change log file name on-the-fly
  • suitable for intensive concurrent usage: has internal message queue to avoid threads blocking

How to use

Add NReco.Logging.File package reference and initialize a file logging provider in services.AddLogging (Startup.cs):

services.AddLogging(loggingBuilder => {
    loggingBuilder.AddFile("app.log", append:true);


services.AddLogging(loggingBuilder => {
    var loggingSection = Configuration.GetSection("Logging");

Example of the configuration section in appsettings.json:

"Logging": {
    "LogLevel": {
      "Default": "Debug",
      "System": "Information",
      "Microsoft": "Error"
    "File": {
        "Path": "app.log",
        "Append": "True",
        "MinLevel": "Warning",  // min level for the file logger
        "FileSizeLimitBytes": 0,  // use to activate rolling file behaviour
        "MaxRollingFiles": 0  // use to specify max number of log files

Rolling File

This feature is activated with FileLoggerOptions properties: FileSizeLimitBytes and MaxRollingFiles. Lets assume that file logger is configured for "test.log":

  • if only FileSizeLimitBytes is specified file logger will create "test.log", "test1.log", "test2.log" etc
  • use MaxRollingFiles in addition to FileSizeLimitBytes to limit number of log files; for example, for value "3" file logger will create "test.log", "test1.log", "test2.log" and again "test.log", "test1.log" (old files will be overwritten).
  • if file name is changed in time (with FormatLogFileName handler) max number of files works only for the same file name. For example, if file name is based on date, MaxRollingFiles will limit number of log files only for the concrete date.

Change log file name on-the-fly

It is possible to specify a custom log file name formatter with FileLoggerOptions property FormatLogFileName. Log file name may change in time - for example, to create a new log file per day:

services.AddLogging(loggingBuilder => {
    loggingBuilder.AddFile("app_{0:yyyy}-{0:MM}-{0:dd}.log", fileLoggerOpts => {
        fileLoggerOpts.FormatLogFileName = fName => {
            return String.Format(fName, DateTime.UtcNow);

Note that this handler is called on every log message 'write'; you may cache the log file name calculation in your handler to avoid any potential overhead in case of high-load logger usage.

Custom log entry formatting

You can specify FileLoggerProvider.FormatLogEntry handler to customize log entry content. For example, it is possible to write log entry as JSON array:

loggerFactory.AddProvider(new NReco.Logging.File.FileLoggerProvider("logs/app.js", true) {
    FormatLogEntry = (msg) => {
        var sb = new System.Text.StringBuilder();
        StringWriter sw = new StringWriter(sb);
        var jsonWriter = new Newtonsoft.Json.JsonTextWriter(sw);
        return sb.ToString();

(in case of .NET Core 2 use loggingBuilder.AddProvider instead of loggerFactory.AddProvider).


Copyright 2017-2020 Vitaliy Fedorchenko and contributors

Distributed under the MIT license

*Note that all licence references and agreements mentioned in the NReco.Logging.File README section above are relevant to that project's source code only.