Is there any way to do a conditional in a NLog layout?

Question

I'm trying to create a NLog layout config like so:

layout = "${callsite} > ${message} (${exception:format=tostring})"

The output for this layout is (when I test it):

TestProject.Program.Main > exception thrown (System.Exception: exception message)

However, if I try to output a message that doesn't have an exception, it shows up like this:

TestProject.Program.Main > no exception thrown ()

Is there any way to modify this layout so that the parenthesis only show up when there's actually an exception?

Answer 1

You could write you own LayoutRenderer like this:

  [LayoutRenderer("MyCustomExceptionLayoutRenderer")]
  class MyCustomExceptionLayoutRenderer : LayoutRenderer
  {
    protected override void Append(StringBuilder builder, LogEventInfo logEvent)
    {
      if (logEvent.Exception == null)
      {
        builder.Append("no exception thrown");
      }
      else
      {
        //Might want fancier formatting of exception here...
        builder.Append("exception thrown ({0})", logEvent.Exception.ToString());
      }
    }

    protected override int GetEstimatedBufferSize(LogEventInfo logEvent)
    {
      return 100;
    }
  }

I think (but have not tried it) that you could also define your LayoutRenderer to take one (or more parameters) like this:

  public enum BracketOption
  {
    None,
    Parentheses,        // ()
    CurlyBraces,        // {}
    SquareBraces,       // []
    LessThanGreaterThan // <>
  }

  [LayoutRenderer("MyCustomExceptionLayoutRenderer")]
  class MyCustomExceptionLayoutRenderer : LayoutRenderer
  {
    public BracketOption Option { get; set; }

    private string left = "";
    private string right = "";

    public MyCustomExceptionLayoutRenderer()
      :base()
    {
      Option = BracketOption.None;
    }

    protected override void Initialize()
    {
      switch(Option)
      {
        case BracketOption.Parentheses:
          left = "(";
          right = ")";
          break;
        case BracketOption.CurlyBraces:
          left = "{";
          right = "}";
          break;
        case BracketOption.SquareBraces:
          left = "[";
          right = "]";
          break;
        case BracketOption.LessThanGreaterThan:
          left = "<";
          right = ">";
          break;
        case BracketOption.None:
        default:
          left = "";
          right = "";
          break;
      }
    }

    protected override void Append(StringBuilder builder, LogEventInfo logEvent)
    {
      if (logEvent.Exception == null)
      {
        builder.Append("no exception thrown");
      }
      else
      {
        //Might want fancier formatting of exception here...
        builder.Append(string.Format("exception thrown {0}{1}{2}", 
                        left, logEvent.Exception.ToString(), right);
      }
    }

    protected override int GetEstimatedBufferSize(LogEventInfo logEvent)
    {
      return 100;
    }
  }

To use, add lines like this to NLog.config (or app.config if configuring inline):

<extensions>
  <add assembly="MyNLogExtensions" />
</extensions>

And add to your layout like this:

<targets>
  <target type="Console" layout="Exception: ${MyCustomException}" />
</targets>

The parameterized layout might look like this (not sure of exact format for specifying options):

<targets>
  <target type="Console" layout="Exception: ${MyCustomException:BracketOption=Parentheses}" />
</targets>