NLog extensibility - How to add custom field using ExtendValues? - asp.net-mvc

Question

I try to add some custom fields to NLog using extensibility.
Part of my nlog.config file looks like that : (simplified for exhibit)
<nlog>
<extensions>
<add assembly="Logzio.DotNet.NLog"/>
</extensions>
<variable name="currentUser" value="test" />
<targets async="true">
<target name="logzio" type="Logzio" token="myToken">
<contextproperty name="currentUser" layout="${currentUser}" />
</target>
</targets>
<rules>
<logger name="*" minlevel="Debug" writeTo="logzio" />
</rules>
</nlog>
In every controller, I have something like that (I'm using ASP.NET MVC5)
private static Logger logger = LogManager.GetCurrentClassLogger();
Then I send my logs to logzio using
logger.Fatal("Something bad happens");
Right now, currentUser always have the value test, which is logical.
However, despite of the documentation, I don't understand how to dynamically change currentUser value by the ID of my current logged user.
Should I create a sort of factory ? (if yes, how ? I'm not at ease with factories)
Should I change my logger variable ? If so, how ?
A piece of code would be extremly welcome.
Thank you for pointing my out where I'm wrong
EDIT
After #Rolf's answer, I've created this custom layout renderer
[LayoutRenderer("custom-layout")]
public class CustomLayoutRenderer : LayoutRenderer
{
public string IdUser { get; set; }
protected override void Append(StringBuilder builder, LogEventInfo logEvent)
{
logEvent.Properties.Add("currentUser", "HowToPassCustomDataHere?");
builder.Append("test from custom-layout");
}
}
and I changed the nlog.config accordingly, adding
layout="${message} ${custom-layout}"
to my <target>
However, I still don't understand how to pass custom value to currentUser. In logz.io, I have "HowToPassCustomDataHere?" as a value of currentUser.
(BTW, ${aspnet-user-identity} is great and works fine ; however I'd like to understand how to pass a custom value to my layout renderer. In my case, something like ${aspnet-user-id})

Answer 1

You can try one of these NLog layoutrenderers to acquire the current username:
${aspnet-user-identity} Wiki
${windows-identity} Wiki
You can also create your own custom NLog LayoutRenderer: https://github.com/NLog/NLog/wiki/How-to-write-a-custom-layout-renderer
Example of how to provide it as currentUser:
<target name="logzio" type="Logzio" token="myToken">
<contextproperty name="currentUser" layout="${aspnet-user-identity}" />
</target>