为什么自定义成员资格提供程序文件必须位于我的Web应用程序的app_code文件夹中？

I'm writing a custom memberhsip provider based on the source for the SQL provider and I tried first making the SQL Provider a separate project. Yet, the web app wouldn't recognize it unless I put all the source in an APP_Code folder in my web app.I'm writing a custom memberhsip provider based