Question

I’m using NLog and logging events to SQL server. One of the fields logged is the log level, this is logged as a string, ‘Info’, ‘Warn’ etc. I also want to log the level as an integer value to enable me to provide better sorting when viewing the log events in a GUI.

Is it possible to convert enums to integers for inserting into a database without writing a custom layout renderer?

The configuration line to log the level into a database as a string is

<parameter name="@Level" layout="${level}"/>

Ideally there might be a conversion to integer, like;

<parameter name="@LevelId" layout="${level:format=tointeger}"/>

much like there is a format=tostring converter.

Answer 1

Here is a custom LayoutRenderer that I wrote for just this purpose. I wrote it against NLog 1.0, so it might not fully adhere to NLog 2.0 conventions for LayoutRenderer (e.g. I think that GetEstimatedBufferSize is no longer used):

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

using NLog;
using NLog.LayoutRenderers;

namespace NLog.Extensions
{
  [LayoutRenderer("LogLevelOrdinal")]
  class LogLevelOrdinalLayoutRenderer : LayoutRenderer
  {
    IDictionary<LogLevel, int> ordinals;

    public override void  Initialize()
    {
      base.Initialize();

      ordinals = GetLogLevels()
                  .OrderBy(level => level)
                  .Select((level, index) => new { Level = level, Ordinal = index })
                  .ToDictionary( x => x.Level, x => x.Ordinal);
    }

    protected override void Append(StringBuilder builder, LogEventInfo logEvent)
    {
      int level = 0;

      if (!ordinals.TryGetValue(logEvent.Level, out level)) level = 99;

      builder.Append(level);
    }

    protected override int GetEstimatedBufferSize(LogEventInfo logEvent)
    {
      return 1;
    }

    //
    // LogLevel is a static class with a static member for each of the different log levels.
    // The LogLevel ordinal is not exposed publically (i.e. an ordinal indicating the relative
    // "importance" of a LogLevel value).
    // The implementation of LogLevel.GetHashCode actually does return the ordinal, but it doesn't
    // seem right to rely on that implementation detail.
    // In the end, this LayoutRenderer is really just to allow for producing a numeric value to represent
    // a particular log level value's "position" relative to the other lob levels.  As such, 
    // We can just get all of the known log level values, order them (they are sortable), and assign our
    // own ordinal based on the position of the log level value in the resulting sorted list.
    //
    // This helper function exposes the known log level values as IEnumerable<LogLevel> so that we can
    // easily use LINQ to build a dictionary to map LogLevel to ordinal.
    //
    internal IEnumerable<LogLevel> GetLogLevels()
    {
      yield return LogLevel.Trace;
      yield return LogLevel.Debug;
      yield return LogLevel.Info;
      yield return LogLevel.Warn;
      yield return LogLevel.Error;
      yield return LogLevel.Fatal;
    }

  }
}