A custom Log4Net Appender that outputs log statements to the Unity console.

Bootstrap.cs

using System.IO;
using log4net.Config;
using UnityEngine;

public static class Bootstrap
{
    [RuntimeInitializeOnLoadMethod(RuntimeInitializeLoadType.BeforeSceneLoad)]
    private static void ConfigureLogging()
    {
        XmlConfigurator.Configure(new FileInfo($"{Application.dataPath}/log4net.xml"));
    }
}

log4net.xml

<log4net>
    <appender name="UnityAppender" type="UnityAppender">
        <layout type="log4net.Layout.PatternLayout,log4net">
            <param name="ConversionPattern" value="%d{ABSOLUTE} %-5p %c{1}:%L - %m%n"/>
        </layout>
    </appender>
    <root>
        <level value="DEBUG"/>
        <appender-ref ref="UnityAppender"/>
    </root>
</log4net>

UnityAppender.cs

using log4net.Appender;
using log4net.Core;
using UnityEngine;

public class UnityAppender : AppenderSkeleton
{
    protected override void Append(LoggingEvent loggingEvent)
    {
        switch (loggingEvent.Level.Name)
        {
            case "DEBUG":
                Debug.Log(RenderLoggingEvent(loggingEvent));
                break;
            case "ERROR":
                Debug.LogError(RenderLoggingEvent(loggingEvent));
                break;
        }
    }
}