LINQ：分组依据+在哪里在条款

Question

我试图在LINQ中实现where in (select ...)代码的T-SQL等效项。

这是我现在有：

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 
        }; 

然而，我的代码不会编译由于这种错误的ToList()：

无法从 'System.Linq.IQueryable<AnonymousType#1>' 转换 到 'System.Collections.Generic.IEnumerable<string>'

A对于如何实现这个，有什么建议吗？或者有关如何简化此代码的任何建议？

Answer 1

您的查询返回一组匿名类型;您不能将其隐式转换为List<string>。

相反，您应该选择字符串本身。你不需要任何匿名类型。

将其更改为

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() 
      };