If you just need to get a list of users, you can use this code -
var dirEntry = new DirectoryEntry(string.Format("LDAP://{0}/{1}", "x.y.com", "DC=x,DC=y,DC=com"));
var searcher = new DirectorySearcher(dirEntry)
{
Filter = "(&(&(objectClass=user)(objectClass=person)))"
};
var resultCollection = searcher.FindAll();
However, if you need more AD operations, you should use the LINQ to AD API http://linqtoad.codeplex.com/
API, Linq, AD. , .