Active Directory Service Interfaces (ADSI) is a programmatic interface for Microsoft Windows Active Directory. It enables your applications to interact with diverse directories on a network, using a single interface. Visual Studio .NET and the .NET Framework make it easy to add ADSI functionality with the DirectoryEntry and DirectorySearcher components.
Using ADSI, you can create applications that perform common administrative tasks, such as backing up databases, accessing printers, and administering user accounts. ADSI makes it possible for you to:
• Log on once to work with diverse directories. The DirectoryEntry component class provides username and password properties that can be entered at runtime and communicated to the Active Directory object you are binding to.
• Use a single application programming interface (API) to perform tasks on multiple directory systems by offering the user a variety of protocols to use. The DirectoryServices namespace provides the classes to perform most administrative functions.
• Perform "rich querying" on directory systems. ADSI technology allows for searching for an object by specifying two query dialects: SQL and LDAP.
• Access and use a single, hierarchical structure for administering and maintaining diverse and complicated network configurations by accessing an Active Directory tree.
• Integrate directory information with databases such as SQL Server. The DirectoryEntry path may be used as an ADO.NET connection string provided that it is using the LDAP provider.
using System.DirectoryServices;

Thanks!
Manoj [InfoAxon Technologies Ltd.]