There is a key parameter for DirectoryEntry ctor()
It is the path, we have to find the root path by check AD User screen.
Like
OU=SYG,OU=CN,OU=Accor-Services,DC=asnet,DC=accorservices,DC=net
LDAP DirectoryEntry access AD User,布布扣,bubuko.com
LDAP DirectoryEntry access AD User
原文：http://www.cnblogs.com/alwaysdotnet/p/3905241.html