Iterate over AD users and find some properties (EmployeeNumber, ExtensionAttributes, manager,..)

using (var principalContext = new PrincipalContext(ContextType.Domain))
{
    var userPrinciple = new UserPrincipal(principalContext);
    // Performe search for Domain users
    using (var searchResult = new PrincipalSearcher(userPrinciple))
    {
        var allUsers = searchResult.FindAll();
           ...... process data....      

    }
}