Single vs. Multiple Value Attributes

Attributes in the directory may contain single or multiple values. A single attribute can contain NULL or a single value. An attribute with multiple values can contain NULL, a single value, or many values.

Note  In Active Directory and most other LDAP servers, the order of an attribute with multiple values is not predetermined. Also, each value in an attribute with multiple values must be unique.

ADSI normally loads schema information if your directory supports a schema, as Active Directory does. Since ADSI knows the syntax of attributes in the schema, you don't need to specify the attribute type when accessing it. ADSI does the hard work for you by marshaling attribute values to the appropriate data type as defined in the schema.

If your directory has no schema, you must supply the data type when you're accessing an attribute.

Note  Active Directory, Exchange, Windows NT 4.0, and Site Server all have a schema. In addition, Active Directory has an extensible schema.