LINQ to Entities 不识别方法“System.String Join(System

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())
            };

 是的，不能认识这个方法。你可以用  AsEnumerable 

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())
            });