LINQ to Entities不识别C#语法报错

错误：报错不识别string.Join……

var QueryWithStandard=from a in listA
          join b in listB
          on a.ID equals b.AID into t
          select new
            {
                ID=a.ID,
                Content=a.Content,
                UserIDs=string.Join(",",t.Select(x=>x.UserID.ToString()).ToArray())
            };

NET4.5+开始；要先 ToList 或者AsEnumerable拿到数据，再使用string语法

正确

var QueryWithStandard=(from a in listA
          join b in listB
          on a.ID equals b.AID into t select new { a, b, t }).AsEnumerable().Select(x =>
                    new {
                ID=x.a.ID,
                Content=x.a.Content,
                UserIDs=string.Join(",",x.t.Select(y=>y.UserID.ToString()).ToArray())
            });