linq 兩個list合並處理,並分組

//兩個list合並處理
/unreadMsgList = unreadMsgList.Union(list).ToList();

 //合並處理[MsgCount]
 unreadMsgList.ForEach(c =>
 {
 var group = unreadMsgList.Where(a => a.SenderMark == c.SenderMark);
 c.MsgCount = group.Sum(x => x.MsgCount);
 });

 //去重復
 unreadMsgList = unreadMsgList.Distinct().ToList();