外部文件中的Log4Net配置不起作用

<appSettings>
    <add key="log4net.Config" value="Log4Net.config"/>
</appSettings>

<?xml version="1.0" encoding="utf-8" ?>
<log4net>
    <appender name="RollingFileAppender" type="log4net.Appender.RollingFileAppender">
        <file value="Logs/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>

log4net.Config.XmlConfigurator.Configure();