Extensibility Model - Extending UserPrincipal

My code:

var context = new PrincipalContext(
  ContextType.Domain, "domain controller", "ou=foo,dc=bar,dc=com");
var userSearcher =
  new PrincipalSearcher(new ActiveDirectoryUserPrincipal(context));
var results = userSearcher.FindAll();
Console.WriteLine(results.Count()); // outputs 2182

var regularUserSearcher = new PrincipalSearcher(new UserPrincipal(context));
var results2 = regularUserSearcher.FindAll();
Console.WriteLine(results2.Count()); // outputs 1579

var computerSearcher = new PrincipalSearcher(new ComputerPrincipal(context));
var results3 = computerSearcher.FindAll();
Console.WriteLine(results3.Count()); // outputs 603


[DirectoryObjectClass("user")]
[DirectoryRdnPrefix("cn")]
public class ActiveDirectoryUserPrincipal : UserPrincipal
{
  ...
}

My custom principal returns computers and users. Anyone have a good suggestion on the best way to fix this? I know I could do this:

DirectoryProperty("objectCategory")]
public string ObjectCategory
{
    get
    {
        var result = this.ExtensionGet("objectCateg开发者_开发知识库ory");
        if (result == null || result.Length != 1)
            return null;
        else
            return (string)result[0];
    }
    set
    {
        this.ExtensionSet("objectCategory", value);
    }
}

and change the search to

var userSearchTemplate = new ActiveDirectoryUserPrincipal(context);
userSearchTemplate.ObjectCategory = "person";
var userSearcher = new PrincipalSearcher(userSearchTemplate);
var results = userSearcher.FindAll();

but it seems more like a hack than a solution...