Since you are only concerned with error level or worse log events, and that traffic is fortunately infrequent, I would suggest configuring your appender to immediately flush.
<param name="ImmediateFlush" value="true" />
This saves you from having to programmatically flush your appender on every log event (which wasn't working anyway from the sound of it). Now, if you wish to open your appender up to more log levels, then of course immediately flushing all events could have greater performance concerns.
EDIT
I added the config file and a simple main program I used for testing. Using the following, I do see log events immediately flushed. In regard to your comment, I can also strip out the ImmediateFlush line from the xml and see the default true value work for flushing. I kept the line in my example for purposes of explicitly stating the desired behavior.
Basic main prog:
class Program
{
static void Main(string[] args)
{
ILog log = LogManager.GetLogger(typeof(Program));
XmlConfigurator.Configure(new FileInfo(@"C:\temp\logTest.config"));
string msg;
while ((msg = Console.ReadLine()) != "Done")
{
log.Error(msg);
}
LogManager.Shutdown();
}
}
logTest.config referenced by main prog:
<log4net>
<appender name="RollingErrorFileAppender" type="log4net.Appender.RollingFileAppender">
<param name="File" value="C:\temp\log" />
<param name="AppendToFile" value="true" />
<param name="DatePattern" value="_yyyyMMddHH".log"" />
<param name="RollingStyle" value="Date" />
<param name="StaticLogFileName" value="false" />
<param name="ImmediateFlush" value="true" />
<filter type="log4net.Filter.LevelRangeFilter">
<param name="LevelMin" value="ERROR" />
<param name="LevelMax" value="FATAL" />
</filter>
<layout type="log4net.Layout.PatternLayout">
<param name="ConversionPattern" value="%utcdate{yyyy-MM-dd HH:mm:ss.fff},[%thread],%level,%logger,%m%n"/>
</layout>
</appender>
<root>
<level value="INFO" />
<appender-ref ref="RollingErrorFileAppender" />
</root>
</log4net>
