list all local users using directory services

The following method I created seem does not work. An error always happens on foreach loop.

NotSupportedException was unhandled...The provider does not support searching and cannot search WinNT://WIN7,computer.

I'm querying the local machine

 private static void listUser(string computer)
 {
        using (DirectoryEntry d= new DirectoryEntry("WinNT://" + 
                     Environment.MachineName + ",computer"))
        {
           DirectorySearcher ds = new DirectorySearcher(d);
            ds.Filter = ("objectClass=user");
            foreach (SearchResult s in ds.FindAll())
            {

              //display name of each user

            }
        }
    }