using (var context = new PrincipalContext(ContextType.Domain, "yourdomain.com"))
{
    using (var searcher = new PrincipalSearcher(new UserPrincipal(context)))
    {
        foreach (var result in searcher.FindAll())
        {
            DirectoryEntry de = result.GetUnderlyingObject() as DirectoryEntry;
            Console.WriteLine("First Name: " + de.Properties["givenName"].Value);
            Console.WriteLine("Last Name : " + de.Properties["sn"].Value);
            Console.WriteLine("SAM account name   : " + de.Properties["samAccountName"].Value);
            Console.WriteLine("User principal name: " + de.Properties["userPrincipalName"].Value);
            Console.WriteLine();
        }
    }
}
Console.ReadLine();