How to activate logging in runtime without restarting application?

One of the main features of NLog is that you could change the config while running the application.

If you have a nlog.config, you could enable autoreload:

<nlog autoReload="true">
   ...
</nlog>

Then the config will be reloaded when you save the changes to the file.

Example

For example:

In the config you could change the globalThreshold or the <rules> while running.

<?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"
      globalThreshold="Debug" >

  <!-- the targets to write to -->
  <targets>
    <!-- write logs to file  -->
    <target xsi:type="File" name="file1" fileName="c:\temp\nlog-all-${shortdate}.log"
            layout="${longdate}|${event-properties:item=EventId_Id}|${uppercase:${level}}|${logger}|${message} ${exception:format=tostring}" />

  </targets>

  <!-- rules to map from logger name to target -->
  <rules>
    <logger name="*" minlevel="Trace" writeTo="file1" />
  </rules>

</nlog>

From code

You could also change the config from code, then check LogManager.Configuration. Please note that this property is null if you haven't a config yet (from code or nlog.config).