This example causes the messages not contains the string '1' to be ignored.

To set up the target in the configuration file, use the following syntax:

<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd" 
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"> 
  <targets> 
    <target name="file" xsi:type="FilteringWrapper" 
            condition="contains('${message}','1')"> 
      <target xsi:type="File" fileName="${basedir}/file.txt"/> 
    </target> 
  </targets> 
  <rules> 
    <logger name="*" minlevel="Debug" writeTo="file"/> 
  </rules> 
</nlog> 

The above examples assume just one target and a single rule. See below for a programmatic configuration that's equivalent to the above config file:

using System; 
 
using NLog; 
using NLog.Targets; 
using NLog.Targets.Wrappers; 
using System.Diagnostics; 
 
class Example 
{ 
    static void Main(string[] args) 
    { 
        FileTarget wrappedTarget = new FileTarget(); 
        wrappedTarget.FileName = "${basedir}/file.txt"; 
 
        FilteringTargetWrapper filteringTarget = new FilteringTargetWrapper(); 
        filteringTarget.WrappedTarget = wrappedTarget; 
 
        filteringTarget.Condition = "contains('${message}','1')"; 
 
        NLog.Config.SimpleConfigurator.ConfigureForTargetLogging(filteringTarget, LogLevel.Debug); 
 
        Logger logger = LogManager.GetLogger("Example"); 
        logger.Debug("log message 0"); 
        logger.Debug("log message 1"); 
        logger.Debug("log message 2"); 
        logger.Debug("log message 11"); 
    } 
}