LINQ to SQL - Group By/Where

I'm trying to implement a group messaging feature in ASP.NET MVC. I want to display a list of all threads for a specific ContactID, displaying the latest message in that thread (no matter who it's from). I've set up my table as below:

MessageID ThreadID MessageBody ContactID
10000004 300152,300160, msg1 300160
10000005 300152,300160, msg2 300160
10000008 300152,300160, msg3 300152

I was able to display the latest message grouped by ThreadID. Ex:

ThreadID Count LatestMessage
300152,300160, 3 10000008

However, if I add the Where clause before the group by (see below), it'll filter on ContactID first before doing the group by, and producing this result:

ThreadID Count LatestMessage
300152,300160, 2 10000005

Here's the code:

        var result = from s in pdc.Messages
                     where s.ContactID == contactID
                     group new { s } by new { s.ThreadID } into d
                     let maxMsgID = d.Max(x => x.s.Mes开发者_StackOverflow中文版sageID)
                     select new {
                         ThreadID = d.Key.ThreadID,
                         Count = d.Count(item => item.s.MessageType == GlobalConstants.MessageTypeText),
                         LastMessage = d.Where(x => x.s.MessageID == maxMsgID)
                     };

Is there a way to do the group by and then filter on ContactID?

    var result = from s in pdc.Messages
                 where s.ContactID == contactID
                 group new { s } by new { s.ThreadID } into d
                 select new {
                     ThreadID = d.Key,
                     Count = d.Count(item => item.s.MessageType == GlobalConstants.MessageTypeText),
                     LastMessage = d.select(x => x.s.MessageID).Max(),
                 };

Hope it can help you and have a nice day.