Accessing the Directory: CDO or ADSI?

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The big question is how to access the directory provided by Exchange Server. Consider using CDO to access the Exchange Server directory when you are running earlier versions of Exchange Server such as Exchange Server 4.0, because Exchange Server 4.0 does not support LDAP, which is required to support the ADSI object library. CDO offers you a limited subset of features because it can access only the properties that have corresponding messaging API (MAPI) unique IDs. Therefore, if CDO provides access to the directory properties that you need in your application, you should use it for directory access. Using CDO has the benefit of allowing you to write your application using a familiar object library, and you do not have to write to a second object library, which can save debugging time.

The other way to access the Exchange Server directory is by using the ADSI object library, which gives you more flexibility. Using ADSI, you can access not only the properties used by CDO but also other properties stored in the Exchange Server directory—for example, configuration information, such as which connectors are installed on the Exchange Server, and whether the Exchange Server is anonymously publishing its directory entries to http clients.

To use ADSI, you have to run a version of Exchange Server that supports LDAP. Exchange Server 5.0 supports LDAP read-only access, and Exchange Server 5.5 supports LDAP read/write access. I recommend that if you want to use ADSI with your Exchange Server, you should install or upgrade to Exchange Server 5.5. With CDO, you can use any version of Exchange Server, from version 4.0 through version 5.5.