NLog trace target only works right for trace and debug levels, not info, warn, error, or fatal

Question

_{Please note I am answering my own question to help people in the future.}

When I use NLog to log to a trace target type, it only works correctly if the level is Trace or Debug . If it's Info , Warn , or Error , it prepends extra info before my layout. If the level is Fatal , I get a pop-up complaining about a failed assertion and it says so in the output too. Is the trace target bugged?

Program.cs

using System;

namespace TraceLoggingTest
{
    class Program
    {
        private static NLog.Logger logger = NLog.LogManager.GetCurrentClassLogger();

        static void Main(string[] args)
        {
            logger.Trace("Hello Trace");
            logger.Debug("Hello Debug");
            logger.Info("Hello Info");
            logger.Warn("Hello Warn");
            logger.Error("Hello Error");
            logger.Fatal("Hello Fatal");

            Console.ReadLine();
            NLog.LogManager.Shutdown();
        }
    }
}

NLog.config

<?xml version="1.0" encoding="utf-8" ?>
<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:schemaLocation="http://www.nlog-project.org/schemas/NLog.xsd NLog.xsd"
      autoReload="true"
      throwExceptions="false"
      internalLogLevel="Off" internalLogFile="c:\temp\nlog-internal.log">

  <targets>
    <target name="logTrace" xsi:type="Trace" layout="${message}" />
  </targets>

  <rules>
    <logger name="*" minlevel="Trace" writeTo="logTrace" />
  </rules>
</nlog>

Expected output

Hello Trace
Hello Debug
Hello Info
Hello Warn
Hello Error
Hello Fatal

Actual output

Hello Trace
Hello Debug
TraceLoggingTest.exe Information: 0 : Hello Info
TraceLoggingTest.exe Warning: 0 : Hello Warn
TraceLoggingTest.exe Error: 0 : Hello Error
---- DEBUG ASSERTION FAILED ----
---- Assert Short Message ----
Hello Fatal
---- Assert Long Message ----

Answer 1

A fix for the prepending issue was introduced in Nlog 4.5. You need to add rawWrite="true" to the target statement. It also happens to fix the assertion problem.

Change:

<target name="logTrace" xsi:type="Trace" layout="${message}" />

To this:

<target name="logTrace" xsi:type="Trace" layout="${message}" rawWrite="true" />

From https://github.com/NLog/NLog/wiki/Trace-target :

Layout Options

rawWrite - Always use Trace.WriteLine independent of LogLevel. Default: False

Introduced with NLog 4.5, fixes the issue with output being prefixed with exe-filename.

Here are two discussions of the original problem:

• https://github.com/NLog/NLog/issues/1968
• https://github.com/NLog/NLog/issues/2234