如何在log4net中过滤自定义级别？

public class VerboseLevel : Level
{
    public static readonly VerboseLevel Verbose = new VerboseLevel(50000, "VERBOSE");

    public VerboseLevel(int level, string name) : base(level, name)
    {
    }
}

<log4net>
  <appender name="RollingFileAppender" type="log4net.Appender.RollingFileAppender">
    <file value="log.txt" />
   <appendToFile value="true" />
   <rollingStyle value="Size" />
    <maxSizeRollBackups value="10" />
   <maximumFileSize value="10MB" />
   <staticLogFileName value="true" />
   <layout type="log4net.Layout.PatternLayout">
      <conversionPattern value="%date [%thread] %-5level %logger - %message%newline" />
    </layout>
   <filter type="log4net.Filter.LevelRangeFilter">
     <levelMin value="VERBOSE" />
     <levelMax value="FATAL" />
    </filter>
  </appender>
  <root>
   <level value="VERBOSE" />
    <appender-ref ref="RollingFileAppender" />
  </root>
</log4net>

private static readonly ILog log = LogManager.GetLogger(typeof(Program));

static void Main(string[] args)
{
    log4net.Config.XmlConfigurator.Configure();

    log.Verbose("This is a VERBOSE level message");
    log.Debug("This is a DEBUG level message");
    log.Info("This is an INFO level message");
    log.Warn("This is a WARN level message");
    log.Error("This is an ERROR level message");
    log.Fatal("This is a FATAL level message");
}