Develop Reference

how to use etw nlog for writing log to custom eventsource

For my console application , I want to write all the logs to custom event source under Application and Services Logs under separate section MyEventSourceName.
I tried to use NLog.Etw, but seems nothing appears. How to do this?
<?xml version="1.0" encoding="utf-8" ?>
<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
autoReload="true"
throwExceptions="false">
<extensions>
<add assembly="NLog.Etw" />
</extensions>
<targets async="true">
<target xsi:type="EtwEventSource"
name="eetw"
providerName="MyEventSourceName"
taskName="${level}"
layout="${message}">
</target>
</targets>
<rules>
<logger name="*" minlevel="Debug" writeTo="eetw" />
</rules>
class Program
{
private static Logger logger = LogManager.GetCurrentClassLogger();
static void Main(string[] args)
{
logger.Info("New person created with name {0}");
}
}

You could use the eventlog target:
For example:
<target xsi:type="EventLog"
name="eventlog"
source="MyEventSourceName"
log="MyEventSourceLogName"
layout ="${message}${newline}${exception:format=ToString}"/>
See docs
The target supports .NET3.5+ and .NET Standard 2.0. For NetStandard 2.0 use the NLog.WindowsEventLog package.

NLog 2.1 EventId for EventLog not working when not specified

I am using Nlog 2.1 and trying to write errors into Windows Event logger with different eventId. To better distinguish different errors.
If I specify eventId property it's working, but if don't I am not seeing any record in Windows Event Logger.
NLog.config file:
<?xml version="1.0" encoding="utf-8" ?>
<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<targets>
<target name="console" xsi:type="ColoredConsole"
layout="${date:format=HH\:mm\:ss}|${level:uppercase=true}|${message}" />
<target xsi:type="EventLog"
name="eventlog"
layout="{${newline}
"Logger": "${logger}",${newline}
"StackTrace": "${stacktrace}",${newline}
"Message": "${message}",${newline}
"Exception": "${exception:format=ToString,Data}"${newline}}"
machineName="."
source="CareFusion Analytics Agent Service"
eventId="${event-properties:EventID}"
log="Application" />
</targets>
<rules>
<logger name="*" minlevel="Trace" writeTo="console" />
<logger name="*" minlevel="Error" writeTo="eventlog" />
</rules>
</nlog>
Usage:
private static void Main(string[] args)
{
Logger logger = LogManager.GetCurrentClassLogger();
logger.Error("Sample error message"); //This is not working
LogEventInfo logEvent = new LogEventInfo()
{
Level = LogLevel.Error,
Message = "Hello",
LoggerName = logger.Name
};
logEvent.Properties.Add("EventID", 400);
logger.Log(logEvent); //This is working
Console.WriteLine("Press any key....");
Console.ReadKey();
}

The call logger.Error("Sample error message"); goes wrong as the EventLogTarget tries to convert the ${event-properties:EventID} to a integer.
Because layout renders in NLog never return null (but string.Empty), this will give a exception - which will be caught by NLog. In the NLog's internal log you should see a FormatException.
So we need to specify a default value, you could do that with the whenEmpty:
<target xsi:type="EventLog"
...
eventId="${event-properties:EventID:whenEmpty=0}" />
PS: tested it with NLog 4.3.11

Log only to a specific target at runtime

I am using NLog with two targets:
<?xml version="1.0" encoding="utf-8" ?>
<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<targets async="true">
<target name="logfile" xsi:type="File" fileName="my.log"/>
<target name="console" xsi:type="Console"/>
</targets>
<rules>
<logger name="*" minlevel="Trace" writeTo="logfile"/>
<logger name="*" minlevel="Info" writeTo="console"/>
</rules>
</nlog>
Is it possible to log a message only to the "logfile" target, without having the message written to the "console" target as well?
EDIT
To clarify: I want to direct messages from the same class to different loggers at run time (w/o having to change the XML). Something like:
class Program
{
static Logger _logger = LogManager.GetCurrentClassLogger();
static void Main(string[] args)
{
// Note - _logger.InfoToTarget() does not really exist
_logger.InfoToTarget("logfile", "This is my very detailed message, possibly with object dumps");
_logger.InfoToTarget("console", "Short message");
}
}
I'm aware that this couples my code with the NLlog.config file.

One way to accomplish the functionality you are looking for is to name your logger
_logger = LogManager.GetLogger("MyConsoleLogger")
_logger.Info("This will log to the console...");
_logger = LogManager.GetLogger("MyFileLogger")
_logger.Trace("This will log to a file...");
rather than using
LogManager.GetCurrentClassLogger().
In your config file you could then list in the rules
<rules>
<logger name="MyFileLogger" minlevel="Trace" writeTo="logfile"/>
<logger name="MyConsoleLogger" minlevel="Info" writeTo="console"/>
</rules>
This is by far not the most pretty solution to look at, but it does give you the functionality that you are looking for.

There are a few ways to do this, and the correct method depends on your situation.
Keep in mind that you typically want to avoid having your app know too much about the inner-workings of logging. If possible, it's best to configure nlog to decide where things should get logged.
Is there a specific namespace that should not be logged to console? That's easy to configure. Also, you can use the "When" filter (https://github.com/nlog/nlog/wiki/When-filter) or conditions (https://github.com/nlog/nlog/wiki/Conditions)
It may also be best to have multiple logger instances, so you can call the one that is appropriate for each situation (logger per class) (Why do loggers recommend using a logger per class?).

Absolutely, however I am assuming you mean at release you no longer wish to log to the console. You can do this very easily by removing or commenting out the listener that writes to the console target. Now it will only write to the logfile target.
<?xml version="1.0" encoding="utf-8" ?>
<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<targets async="true">
<target name="logfile" xsi:type="File" fileName="my.log"/>
<target name="console" xsi:type="Console"/>
</targets>
<rules>
<logger name="*" minlevel="Trace" writeTo="logfile"/>
<!--<logger name="*" minlevel="Info" writeTo="console"/>-->
</rules>
</nlog>
The rule that writes to the console is now deactivated, but the log file is active. If this is during release you probably want to change your rule to not process your trace logging as the min level for the log file since it will slow down your app with excessive IO. I have asked this question in the past and it appears that best practice is to do this via the XML configuration files. (Logging in Release Build of Application (C#))

How to make NLog terminate application on fatal errors?

Can I somehow set up an event handler for fatal errors in the code? I would like to terminate the application immediately if such an error happens, i.e. something like this:
void Fail(string format, params object[] args) {
Logger.Fatal(format, args);
Environment.Exit(-1);
}
However I would like this to happen automatically:
Logger.Fatal(...); // logs the error and invokes Environment.Exit(-1);
Is there a way to setup some kind of callback for all fatal errors or some configuration option for this?

Yes,
you can do this using MethodCall target (see https://github.com/nlog/NLog/wiki/MethodCall-target)
Here example:
namespace SomeNamespace
{
using System;
public class MyClass
{
public static void HandleFatalEventLog(string level)
{
if(level.Equal("Fatal", StringComparison.OrdinalIgnoreCase))
{
Environment.Exit(-1);
}
}
}
}
And NLog.config file:
<?xml version="1.0" ?>
<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<targets>
<target name="m" xsi:type="MethodCall" className="SomeNamespace.MyClass, MyAssembly" methodName="HandleFatalEventLog">
<parameter layout="${level}" />
</target>
</targets>
<rules>
<logger name="*" minlevel="Fatal" writeTo="m" />
</rules>
</nlog>
PS: But, I would not recommend this approach, logger should not shutdown application

NLog Refuse to Throw Exception

Hello dear developers,
I encountered a weird problem with NLog,
I had a crash, but couldn't find a trace for the user activity in the logs.
i assume, Logging is not working...
tried searching for permissions problems and such, but all seems to be O.K.
I want to debug my logging in-case of problems so i created the following
code to tell the user if it failed to create anything:
public static class Log
{
static Log()
{
try
{
AppLogger = LogManager.GetLogger("Megatec.EngineeringMatrix.AppLogger");
ChangeLogger = LogManager.GetLogger("Megatec.EngineeringMatrix.ChangeLogger");
DRCLogger = LogManager.GetLogger("Megatec.EngineeringMatrix.DRCLogger");
if((AppLogger == null) || (ChangeLogger == null) || (DRCLogger == null))
throw new NLogConfigurationException("Configuration does not specify correct loggers");
writeStartupLogEntry();
}
catch (Exception ex)
{
MessageBox.Show(ex.ToString(), #"Failed to load `Megatec.EngineeringMatrix` Loggers.");
throw;
}
}
public static readonly Logger AppLogger;
public static readonly Logger ChangeLogger;
public static readonly Logger DRCLogger;
with the following configuration file:
<?xml version="1.0" encoding="utf-8" ?>
<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" throwExceptions="true">
<targets>
<target xsi:type="File" name="AppLogFile" createDirs="true" fileName="c:/log?s/${processname}/${logger}_${shortdate}.log"
layout=">> ${time} ${uppercase:${level}} ${callsite} ${message} ${exception:format=tostring}" />
<target xsi:type="File" name="ChangeLogFile" createDirs="true" fileName="c:/log?s/${processname}/${logger}_${shortdate}.log"
layout=">> ${date} ${message} ${exception:format=tostring}" />
<target xsi:type="File" name="DRCLogFile" createDirs="true" fileName="c:/logs/${processname}/${logger}_${shortdate}.log"
layout=">> ${date} ${message} ${exception:format=tostring}" />
</targets>
<rules>
<logger name="Megatec.EngineeringMatrix.AppLogger" minlevel="Trace" writeTo="AppLogFile" />
<logger name="Megatec.EngineeringMatrix.ChangeLogger" minlevel="Trace" writeTo="ChangeLogFile" />
<logger name="Megatec.EngineeringMatrix.DRCLogger" minlevel="Trace" writeTo="DRCLogFile" />
</rules>
</nlog>
OBVIOUSLY I written a BAAD config because a directory c:\lo?gs cannot be created.
but still, i don't get the message
MessageBox.Show(ex.ToString(), #"Failed to load 'Megatec.EngineeringMatrix'
and even AppLogger.Info() skips the exception...
nothing i written to log, but the App don't know it...
In debug, i can catch the exception, and see it is handled by NLog,
How can I catch it from my code?

This is an old issue, but there are some recent changes on this.
Some exceptions were 'eaten' by NLog in the past. For example in the AsyncWrapper (or using the attribute async on <target>)
This has been fixed in NLog 4.3:
Consistent handling of exceptions (BEHAVIOUR CHANGE)
The logging and throwing of exceptions was previously inconsistent. Not all of it was logged to the internal logger and some times they got lost. For example, the async wrapper (or async attribute) was catching all exceptions without a proper rethrow.
This is bad as it is sometimes unclear why NLog isn’t working (got it myself multiple times). This has been fixed in NLog 4.3!
All exceptions are logged to the internal logger
The “throwExceptions” option will be respected in all cases
Advise: disable throwExceptions in production environments! (this the default)
see http://nlog-project.org/2016/04/16/nlog-4-3-has-been-released.html