cannot find HashSet

using System;
using System.Collections;
using System.Collections.Generic;
using System.Text;

namespace msdb
{
    public class dbWordSet : HashTable
    {
    }
}

Compiler error message is

Error	1	The type or namespace name 'HashTable' could
not be found (are you missing a using directive or an
assembly reference?)	C:\Documents and
Settings\austiny\My Documents\Visual Studio
2005\Projects\MicroServer\msdb\dbWordSet.cs	8	30	msdb

if I change HashTable to List, there is no error
message.