AD request with C#

Question

I'm trying to get all users from AD and their passwords expiration dates with the following code, but got the error.

System.InvalidOperationException: 'The Principal object must be persisted before this method can be called.'

The code:

PrincipalContext domain = new PrincipalContext(ContextType.Domain,"AAA","OU=USERS,OU=TEST,DC=AAA,DC=AA, DC=A");
UserPrincipal user = new UserPrincipal(domain);
PrincipalSearcher pS = new PrincipalSearcher(user);

foreach (UserPrincipal result in pS.FindAll())
    {
    if (result != null && result.DisplayName != null)
        {
        DirectoryEntry entry = (DirectoryEntry)user.GetUnderlyingObject();
        IADsUser native = (IADsUser)entry.NativeObject;
        Console.WriteLine(result.DisplayName, native.PasswordExpirationDate);
        }
     }