Question: In my application, I work on several thousand of document a day. I'd like, in some cases some logs, one log by document. Then I'd like for a specific target change the output filename (and only the filename) at runtime.

Around the web I found how to create a target by programming me I'd like just update a the filename by programming. I tried the code below. The error I receive is "LayoutRender cannot be found 'logDirectory'.

Any idea ?

Thanks,

```
var target = (FileTarget)LogManager.Configuration.FindTargetByName("logfile");
target.FileName = "${logDirectory}/file2.txt";

LoggingConfiguration config = new LoggingConfiguration();
var asyncFileTarget = new AsyncTargetWrapper(target);
config.AddTarget("logfile", asyncFileTarget);

LogManager.Configuration = config;

```

The config file is :

```
  <nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
    <variable name="logDirectory" value="C:/MyLogs"/>
    <targets>
      <target name="logfile" xsi:type="File" layout="${date:format=dd/MM/yyyy HH\:mm\:ss.fff}|${level}|${stacktrace}|${message}" fileName="${logDirectory}/file.txt" />
    </targets>

    <rules>
      <logger name="*" minlevel="Info" writeTo="logfile" />
    </rules>    
  </nlog>

```

Answer: 
If not needing to change the LogDirectory at runtime, then you can do this:

```
target.FileName = "${var:logDirectory}\\file2.txt");

```

If needing to modify the logDirectory at runtime, then use the GDC:

<https://github.com/NLog/NLog/wiki/Gdc-layout-renderer>

```
NLog.GlobalDiagnosticsContext.Set("logDirectory","C:\Temp\");

```

Then you can use the following layout:

```
target.FileName = "${gdc:item=logDirectory}\\file2.txt";

```
For anyone stuck on this, I finally found a solution. I was trying to update some syslog target settings at runtime and nothing was working. Simply updating the configuration does not work, you need to reset the Configuration object, which is as simple as doing this:

```
LogManager.Configuration = LogManager.Configuration;

```

This causes an internal event to fire and actually use the updated configuration.