NLog 6.0 - List of major changes

29 Apr 2025

NLog 6.0 is a major version release, and introduces breaking changes to support AOT-builds by splitting into multiple nuget packages.

Major changes

NLog supports AOT

NLog v6 now supports Ahead-of-Time (AOT)-builds without warnings, and optimizes the support for build-trimming that was introduced with NLog v5.2.2.

NLog has traditionally used reflection for dynamic discovery of target output requirements. But reflection does not always work well with build-trimming, and before NLog marked itself to be completely excluded from any trimming.

NLog includes many features, and each feature often introduces additional dependencies on the .NET library. This can lead to overhead for AOT builds, as it must include and compile all the relevant source code.

NLog v6 attempts to reduce its footprint by extracting several features into separate nuget-packages:

NLog v6 also no longer depends on System.Xml.XmlReader, but now includes its own basic XmlParser for loading NLog.config files.

NLog v6 still introduces an overhead when compared with just using Console.WriteLine, but now reduced to 5 MByte in comparison to 14 MBytes with NLog v5.

If the NLog.config-file had to be explicitly loaded, then the AOT-build could trim much more, since right now the AOT-build cannot predict what types will be required by the NLog.config-file. But disabling automatic loading of the NLog.config-file is a huge breaking change, and would hurt lots of existing applications.

NLog FileTarget without ConcurrentWrites

NLog FileTarget no longer uses File.Move by default, but instead rolls to the next filename. This is to prevent file-locking issues with other background-applications, or if the log-file is held open by file-viewer.

The new archive-logic:

  • LogFile.txt (Oldest file)
  • LogFile_1.txt
  • LogFile_2.txt (Newest file)

The old archive-logic:

  • LogFile.txt (Newest file)
  • LogFile.1.txt (Oldest file)
  • LogFile.2.txt

NLog FileTarget still support static archive logic with File.Move, but it must be explictly activated by specifying ArchiveFileName. If not using ArchiveFileName but want to revert to old archive-logic with File.Move, then just assign ArchiveFileName="..." to have the same value as FileName="...".

NLog FileTarget no longer has the following options:

  • ConcurrentWrites - Removed because of dependency on global mutex and exotic file-locks.
  • EnableArchiveFileCompression - Removed because of dependency on compression-libraries.
  • CleanupFileName - Removed because it is now implicit.
  • FileNameKind - Removed because it is now implicit.
  • ArchiveFileKind - Removed because it is now implicit.
  • FileAttributes - Removed because of dependency on Windows-only API.
  • ForceManaged - Removed because of dependency on Windows-only API.
  • ConcurrentWriteAttempts - Removed together with ConcurrentWrites.
  • ConcurrentWriteAttemptDelay - Removed together with ConcurrentWrites.
  • ForceMutexConcurrentWrites - Removed together with ConcurrentWrites.
  • NetworkWrites - Replaced by KeepFileOpen.
  • ArchiveOldFileOnStartupAboveSize - Instead use ArchiveAboveSize / ArchiveOldFileOnStartup.
  • ArchiveDateFormat - Marked as obsolete. Instead use new ArchiveSuffixFormat
  • ArchiveNumbering - Marked as obsolete. Instead use new ArchiveSuffixFormat (Rolling is unsupported).

If one still requires these options, then one can use the new NLog.Targets.ConcurrentFile-nuget-package. The NLog.Targets.ConcurrentFile-nuget-package is the original NLog FileTarget with all its features and complexity. It is the goal that NLog.Targets.ConcurrentFile-nuget-package should become legacy, but it might be helpful when upgrading to NLog v6.

Alternative options for replacing ConcurrentWrites = true:

  • Use the new nuget-package NLog.Targets.AtomicFile where AtomicFileTarget uses atomic file-appends and supports Windows / Linux with NET8.
  • Change to use KeepFileOpen = false where file is opened / closed when writing LogEvents. For better performance then consider to also use <targets async="true">.

Alternative options for replacing EnableArchiveFileCompression = true:

  • Activate NTFS compression for the logging-folder.
  • Setup cron-job / scheduled-task that performs ZIP-compression and cleanup of the logging-folder.
  • Implement background task in the application, which monitors the logging-folder and performs ZIP-compression and cleanup.
  • Use the new nuget-package NLog.Targets.GZipFile where GZipFileTarget writes directly to a compressed log-file using GZipStream.

NLog AtomicFileTarget without mutex

New AtomicFileTarget has been introduced with NLog.Targets.AtomicFile, that supports atomic file-append with help from the operating system, and supports both Windows and Linux (with help from Mono Posix) for NET8 (and newer).

Extends the standard FileTarget and adds support for ConcurrentWrites = true, but without using global mutex.

Linux users must use dotnet publish with --configuration release --runtime linux-x64 to ensure correct publish of the Mono.Posix.NETStandard-nuget-package dependency.

NLog GZipFileTarget with GZipStream

New GZipFileTarget has been introduced with NLog.Targets.GZipFile nuget-package, that writes directly to a compressed log-file using GZipStream.

Extends the standard FileTarget and adds support for EnableArchiveFileCompression = true, but only supports GZip file compression.

The GZip File compression doesn’t allow appending to existing GZip Archive without rewriting the entire GZip Archive. Therefore these options are set by default KeepFileOpen = true and ArchiveOldFileOnStartup = true.

The GZip File compression has better handling of large log-files compared to the old EnableArchiveFileCompression = true, where the old ZIP compression would stall the file-logging until completed.

NLog LogFactory FlushAsync

NLog LogFactory Flush() and Shutdown() are synchronous API methods, that schedules background worker-threads to execute NLog Target Flush. This doesn’t work well on platforms that simulates worker-threads, by running everything on main thread.

Instead NLog LogFactory FlushAsync-method has been introduced that will support multi-threaded flush.

The NLog LogFactory now also implements IDisposableAsync which includes FlushAsync before closing. This allows the following:

await using var logFactory = NLog.LogManager.Setup().LoadConfigurationFromFile().LogFactory; // Automatic Shutdown()

The NLog LogFactory Dispose()-method has been changed to skip flush with help from worker-threads, but will only perform synchronous NLog Target Close.

NLog API with nullable references

NLog API has been updated to enable nullable, and has been compiled using C# v9 for all target-platforms.

NET3 introduced C# v8 with support for nullable reference types, that allows static analysis to avoid NullReferenceException (NRE). NET5 introduced C# v9 with improved support for nullable generics.

This will improve the user-experience, when using the NLog API directly instead of NLog configuration files.

NLog Logger API with ReadOnlySpan

NLog Logger API now supports params ReadOnlySpan and can skip params object[]-allocation, when many parameters and LogLevel is not enabled.

NET9 introduced C# v13 that supports params ReadOnlySpan, which is now supported by NLog for NetStandard2.1.

NLog extends the optimization to completely skip the params object[]-allocation, when using message-templates and it is not possible to defer the parsing of the message-template on background thread.

Legacy NLog Logger API reduced priority

NLog Logger API has legacy methods from .NET Framework 1.0 without generics.

NET9 introduced C# v13 that supports [OverloadResolutionPriority(-1)] to change priority of methods, when other methods are available.

NLog Logger API will now promote the generic-methods and ReadOnlySpan-methods, instead of the legacy methods. This will hopefully reduce the friction when finally cleaning up these legacy methods, when not being in use.

NLog GelfTarget and GelfLayout

The NLog.Targets.Network nuget-package also includes support for the Graylog Extended Log Format (GELF).

The GelfTarget extends the standard NetworkTarget with the new GelfLayout.

It depends on the builtin NLog JSON serializer, but follows the ‘standard’ of prefixing all custom property-names with underscore _.

<nlog>
  <extensions>
    <add assembly="NLog.Targets.Network" />
  </extensions>
  
  <targets async="true">
    <target xsi:type="Gelf" name="GelfTcp" address="tcp://localhost:12200" newLine="true" lineEnding="Null">
      <GelfField name="ThreadId" layout="${ThreadId}" />
    </target>
  </targets>

  <rules>
    <logger name="*" minlevel="Debug" writeTo="GelfTcp" />
  </rules>
</nlog>

Available GelfTarget-options that can be adjusted:

  • GelfHostName
  • GelfShortMessage
  • GelfFullMessage
  • IncludeEventProperties
  • IncludeScopeProperties

NLog SyslogTarget and SyslogLayout

The NLog.Targets.Network nuget-package also includes support for the Syslog Output Format.

The SyslogTarget extends the standard NetworkTarget with the new SyslogLayout.

The SyslogLayout supports both RFC-3164 (simple) + RFC-5424 (structured) logging output.

<nlog>
  <extensions>
    <add assembly="NLog.Targets.Network" />
  </extensions>
  
  <targets async="true">
    <target xsi:type="SysLog" name="SyslogTcp" address="tcp://localhost:514">
      <Rfc3164>false</Rfc3164>
      <Rfc5424>true</Rfc5424>
      <StructuredDataParam name="ThreadId" layout="${ThreadId}" />
    </target>
  </targets>

  <rules>
    <logger name="*" minlevel="Debug" writeTo="SyslogTcp" />
  </rules>
</nlog>

Available SyslogLayout-options that can be adjusted:

  • SyslogAppName
  • SyslogHostName
  • SyslogMessage
  • SyslogLevel
  • SyslogFacility
  • StructuredDataId
  • IncludeEventProperties

NLog NetworkTarget with NoDelay = true

The NLog.Targets.Network nuget-package includes support for configuring TCP_NODELAY. The NetworkTarget will by default use NoDelay = true to turn off delayed ACK, to avoid delays of 200ms because of nagle-algorithm.

Believe most users will not notice the overhead of additional ACK-packages, but will notice a delay of 200ms.

NLog NetworkTarget with SendTimeoutSeconds = 100

The NLog.Targets.Network nuget-package changes the default value of TCP SendTimeout from waiting forever to 100 secs.

The NetworkTarget should now react to the network-cable being unplugged and the TCP send-window being filled.

The NetworkTarget should now automatically attempt to reconnect when the endpoint suddenly becomes unresponsive.

NLog NetworkTarget with SslCertificateFile

The NLog.Targets.Network nuget-package introduces the ability to specify custom SSL certificate from file.

The NetworkTarget now recognizes these new settings:

  • Layout SslCertificateFile
  • Layout SslCertificatePassword

The NetworkTarget can now perform SSL handshake with custom SSL certificate from file, without needing to register the certificate in the global operating-system cache.

Breaking changes

Removed legacy Target-Frameworks

NetStandard 1.3 and NetStandard 1.5 has now been removed, since less relevant as most have moved to NetStandard 2.0 (or newer).

Removes platform support for:

  • NET CoreApp 1.0 + 1.1
  • UAP + UAP10.0 (UWP ver. 1)
  • Tizen30

Considering to also remove NET35 + NET45, since those Target-Frameworks requires extra effort to build when using Visual Studio 2022. Removing old Target-Frameworks will also reduce the file-size of the NLog-nuget-package. If this will break your entire eco-system then Please tell.

NLog Structured Message formatting without quotes

NLog v4.5 introduced support for message-templates, where it followed the Serilog approach by adding quotes around string-values:

Logger.Info("Hello {World}", "Earth"); // Outputs Hello "Earth" with NLog v4.5

Microsoft Extension Logging decided not to implement that behavior, and now NLog also changes to not using quotes by default. Thus avoiding surprises when using NLog Logger directly instead as LoggingProvider for Microsoft Extension Logging.

To apply string-quotes for a single value:

Logger.Info("Hello {@World}", "Earth"); // Outputs Hello "Earth" with NLog v6

It is possible to globally revert to old behavior:

LogManager.Setup().SetupSerialization(s => s.RegisterValueFormatterWithStringQuotes());

NLog JsonLayout EscapeForwardSlash obsolete

NLog v5 changed JsonLayout to have the default value EscapeForwardSlash = false, and now NLog v6 marks the NLog JsonLayout EscapeForwardSlash as completely obsolete and no longer having any effect.

NLog JsonLayout SuppressSpaces default true

The JsonLayout now have the new default value SuppressSpaces = true, since log-file-size / network-traffic-usage doesn’t benefit from the extra spaces.

If the output from JsonLayout needs to be more human readable, then one can explictly assign SuppressSpaces = false or IndentJson = true.

NLog ColoredConsoleTarget with Errors in red

NLog ColoredConsoleTarget used the color Yellow for Errors and Magenta for Warnings. This has now been changed to color Red for Errors and Yellow for Warnings.

This is to align with the normal color standards that most other systems seems to be using.

NLog ColoredConsoleTarget without RegEx

RegularExpressions (RegEx) is a huge API, and a big dependency for a logging library, so to reduce the footprint of NLog then RegEx support have been removed.

This means word-highlighting rules no longer can scan for word-matches using RegEx. But logic has been implemented to continue support IgnoreCase and WholeWords for the string-matching logic.

NLog Replace LayoutRenderer without RegEx

RegularExpressions (RegEx) is a huge API, and a big dependency for a logging library, so to reduce the footprint of NLog then RegEx support have been removed.

Logic has been implemented to continue support IgnoreCase and WholeWords for the string-matching logic.

This means the following has been removed for the ${replace} LayoutRenderer:

  • bool RegEx
  • bool CompileRegex
  • string ReplaceGroupName

If RegEx replace-logic is important then one can use the new nuget-package NLog.RegEx, which includes ${regex-replace}.

NLog InternalLogger without LogToTrace

NLog InternalLogger.LogToTrace has been remnoved. This reduces the NLog footprint by removing references to System.Diagnostics.Trace and System.Diagnostics.TraceListener.

If it is important to redirect NLog InternalLogger output to System.Diagnostics.Trace, then one can use NLog InternalLogger.LogWriter to assign a custom StringWriter that performs the forwarding. Alternative one can setup custom subscriber to NLog InternalLogger.InternalEventOccurred event handler.

NLog XmlParser replaces XmlReader

The .NET System.Xml.XmlReader is a heavy dependency that depend on HttpClient for loading external XML, and depend on IL-code-generation to optimize type serialization. To reduce dependencies and minimize AOT-build-filesize, then NLog now includes its own basic XML-parser.

It could have been nice if Microsoft could refactor System.Xml.XmlReader, so it only introduced a minimal AOT-footprint, but that is probably too late.

The NLog XML-parser only provides basic XML support, but it should be able to load any XML file that was working with NLog v5.

NLog SimpleLayout Immutable

NLog SimpleLayout have removed the setter-method for its Text-property, and is now a sealed class.

This is to simpilfy the NLog SimpleLayout API, and to make it clear that NLog will optimize based on the initial layout.

Many other improvements

For full list of all changes: NLog 6.0 Pull Requests

Comments and feedback are welcome.

Credits

Additional thanks to all contributers, since last major release:

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