In-memory log4net appender, that streams logging events into IObservable<LoggingEvent>. Unlike MemoryAppender, does not persist any logging events by default.
Get Started
$ dotnet add package log4net.ObservableAppenderReadme
Overview 
An appender for log4net, called ObservableAppender which exposes all matched log entries as IObservable<LoggingEvent> which allows to work with logs using System.Reactive framework.
Unlike MemoryAppender, it does not persist any logging events by default and thus does not bloat memory in a long run.
However, it has such an option exposed as Persist property (false by default).
The appender also supports completion of IObservable<LoggingEvent> by logging repository shutdown ar an explicit IAppender.Close() call.
Installation
NuGet package is available here.
PM> Install-Package log4net.ObservableAppender
Examples
Configure log4net with ObservableAppender and subscribe on logging events:
var appender = new ObservableAppender();
Config.BasicConfigurator.Configure(LogManager.GetRepository(Assembly.GetEntryAssembly()), appender);
var logger = LogManager.GetLogger(typeof(ObservableAppenderTests));
appender.LoggingEvents.Subscribe(loggingEvent => ...);
Alternatively, configure log4net by the app config section:
<?xml version="1.0" encoding="utf-8"?>
<configuration>
<configSections>
<section name="log4net" type="log4net.Config.Log4NetConfigurationSectionHandler, log4net" />
<!-- other sections -->
</configSections>
<log4net>
<appender name="ObservableAppender" type="log4net.Appender.ObservableAppender, log4net.ObservableAppender">
<Persist value="true" />
</appender>
<filter type="log4net.Filter.LevelRangeFilter">
<param name="LevelMax" value="FATAL" />
<param name="LevelMin" value="DEBUG" />
</filter>
<layout type="log4net.Layout.PatternLayout">
<conversionPattern value="%-5p %d %5rms %-22.22c{1} %-32.32M - %m%n" />
</layout>
<root>
<appender-ref ref="ObservableAppender" />
</root>
</log4net>
<!-- other sections -->
</configuration>
See more examples at the unit test project.