在log4net中以编程方式更改日志级别？

using log4net;
using log4net.Config;
using log4net.Repository;

<?xml version="1.0" encoding="utf-8" ?>
<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>
  </appender>

  <root>
   <level value="DEBUG" />
    <appender-ref ref="RollingFileAppender" />
  </root>
</log4net>

XmlConfigurator.Configure(new System.IO.FileInfo("log4net.config"));

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

var hierarchy = (Hierarchy)LogManager.GetRepository();
var rootLogger = hierarchy.Root;
rootLogger.Level = Level.Info;
hierarchy.Configured = true;