LINQ: Group By + Where in clause

Question

I'm trying to implement a T-SQL equivalent of a where in (select ...) code in LINQ.

This is what I have now:

int contactID = GetContactID();
IEnumerable<string> threadList = (from s in pdc.Messages
                             where s.ContactID == contactID
                             group 1 by new { s.ThreadID } into d
                             select new { ThreadID = d.Key.ThreadID}).ToList<string>();

        var result = from s in pdc.Messages
                     where threadList.Contains(s.ThreadID)
                     group new { s } by new { s.ThreadID } into d
                     let maxMsgID = d.Where(x => x.s.ContactID != contactID).Max(x => x.s.MessageID)
                     select new {
                         LastMessage = d.Where(x => x.s.MessageID == maxMsgID).SingleOrDefault().s
                     };

However, my code won't compile due to this error for the ToList():

cannot convert from 'System.Linq.IQueryable<AnonymousType#1>' to 'System.Collections.Generic.IEnumerable<string>'

Anyone have any suggestions on how to implement this? Or any suggestions on how to simplify this code?

Solution

Your query returns a set of anonymous types; you cannot implicitly convert it to a List<string>.

Instead, you should select the string itself. You don't need any anonymous types.

Change it to

var threadList = pdc.Messages.Where(s => s.ContactID == contactID)
                             .Select(s => s.ThreadID)
                             .Distinct()
                             .ToList();

var result = from s in pdc.Messages
             where threadList.Contains(s.ThreadID)
             group s by s.ThreadID into d
             let maxMsgID = d.Where(x => x.ContactID != contactID).Max(x => x.MessageID)
             select new {
                 LastMessage = d.Where(x => x.MessageID == maxMsgID).SingleOrDefault()
             };