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) / NET5 / NET6 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 FileLoggerOptions.FormatLogEntry handler to customize log entry content. For example, it is possible to write log entry as JSON array:

loggingBuilder.AddFile("logs/app.js", fileLoggerOpts => {
    fileLoggerOpts.FormatLogEntry = (msg) => {
        var sb = new System.Text.StringBuilder();
        StringWriter sw = new StringWriter(sb);
        var jsonWriter = new Newtonsoft.Json.JsonTextWriter(sw);
        return sb.ToString();

Custom log entry filtering

You may provide a predicate to perform filter log entries filtering on the logging provider level. This may be useful if you want to have 2 (or more) file loggers that separate log entries between log files on some criteria:

loggingBuilder.AddFile("logs/errors_only.log", fileLoggerOpts => {
    fileLoggerOpts.FilterLogEntry = (msg) => {
        return msg.LogLevel == LogLevel.Error;

File errors handling

Log file is opened immediately when FileLoggerProvider is created (= on AddFile call) and you may handle initial file opening errors simply by wrapping AddFile with a try .. catch. However you might want to propose a new log file name to guartee that file logging works even if an original log file is not accessible. To provide your own handling of file errors you may specify HandleFileError delegate:

loggingBuilder.AddFile(loggingSection, fileLoggerOpts => {
    fileLoggerOpts.HandleFileError = (err) => {
        err.UseNewLogFileName( Path.GetFileNameWithoutExtension(err.LogFileName)+ "_alt" + Path.GetExtension(err.LogFileName) );

Real-life implementation may be more complicated to guarantee that a new file name can be used without errors; note that HandleFileError is not recursive and it will not be called if proposed file name cannot be opened too.

A new file name is applied in the same way as when it comes from the initial FileLoggerProvider options (if FormatLogFileName is specified it is called to resolve a final log file name).


Copyright 2017-2022 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.