What about this code? It works quite well:

// initialize, connect and authenticate an IMAP session
Imap imap = new Imap();
...

// select a folder
imap.SelectFolder("Inbox");

// send a search command with free-form text
imap.SendCommand("UID SEARCH", "SUBJECT test SINCE 24-Mar-2009");

// read the response
ImapResponse response = imap.ReadResponse();

// create an empty message set to fill in with message UIDs
ImapMessageSet set = new ImapMessageSet();

// get the folder's validity ID, we will need this later
long validityId = imap.CurrentFolder.ValidityId;

// parse all "SEARCH" responses
foreach (ImapResponseLine line in response.GetLines())
{
    if (line.Code == "SEARCH")
    {
        // get a list of raw IDs
        string[] ids = line.Parameters.Split(' ');
        foreach (string rawId in ids)
        {
            // convert each raw ID into Rebex-style unique ID
            long id = Convert.ToInt64(rawId);
            string uniqueId = ImapMessageSet.BuildUniqueId(validityId, id);

            // add the unique ID into the message set
            set.Add(uniqueId);
        }
    }
}

// use the message set for any purpose

// for example, you can use it to retrieve the list of message envelopes
ImapMessageCollection list = imap.GetMessageList(set, ImapListFields.Envelope);

// diplay message dates and subjects
foreach (ImapMessageInfo info in list)
{
    Console.WriteLine("{0} {1}", info.Date, info.Subject);
}

If you prefer VB.NET, please let us know!