Question

Is it possible to configure NLog to write logs to d:\logs if d: exists, and if d: does not exists then log to c:\logs instead?

Answer 1

I think you could do it with a combination of a custom LayoutRenderer and XML configuration. In XML configuration, you can use LayoutRenderers to specify the filename. For example:

<target name="logfile" xsi:type="File" fileName="${basedir}/${shortdate}.log" />

The FileTarget object will create a file based on the values of the ${basedir} and ${shortdate} layout renderers.

You could easily write a custom LayoutRenderer, maybe call it “FindAvailableDrive”, and configure it like this:

<target name="logfile" xsi:type="File" fileName="${FindAvailableDrive}/logs/${shortdate}.log" />

The FindAvailableDrive LayoutRender could start with a drive letter, say “C:”, and change until it finds a valid drive. To take a step farther, FindAvailableDrive could take one or more parameters that could help define how to search for the valid drive:

<target name="logfile" xsi:type="File" fileName="${FindAvailableDrive:Drives=DC}/logs/${shortdate}.log" />

In this case, FindAvailableDrive would check drive D then drive C.

Here is an example (untested) of how you might write such a LayoutRenderer (note that this is based on an existing LayoutRender that I wrote against NLog 1.0, so some of this might not be necessary (or valid) for NLog 2.0):

  [LayoutRenderer("FindAvailableDrive")]
  class FindAvailableDriveLayoutRenderer : LayoutRenderer
  {
    private string validDrive;

    [DefaultParameter]
    public string DriveCandidates { get; set; }

    protected override void Append(StringBuilder builder, LogEventInfo logEvent)
    {
      builder.Append(FindValidDrive());
    }

    protected override int GetEstimatedBufferSize(LogEventInfo logEvent)
    {
      return 2;
    }

    private string FindValidDrive()
    {
      if (!String.IsNullOrEmpty(validDrive)) return validDrive;

      if (String.IsNullOrEmpty(DriveCandidates))
      {
        if (Directory.Exists("C:"))
          validDrive = "C:";
        else
        if (Directory.Exists("D:"))
          validDrive = "D:";
        //And so on if you want to continue to check for valid drives.
        return validDrive;
      }

      validDrive = DriveCandidates.Select(c => string.Format("{0}:", c).Where(d => Directory.Exists(d)).FirstOrDefault();

      //What to do if no drives exists?

      return validDrive;
    }
  }

You can see my answer to another question here for more information about how to write and use a custom LayoutRenderer in NLog.