关于linq FirstOrDefault()的问题
0
悬赏园豆：10
[已解决问题]
解决于 2013-12-10 13:33
1 var test = (from comment in commentListContext.Comments 2 group comment by comment.CommentActivityID into c 3 select new 4 { 5 activityId = c.Key, 6 commentInfo = (from comment2 in c 7 from activity in commentListContext.Activities 8 where (comment2.CommentActivityID == activity.ActivityID &&
comment2.CommentTime == c.Max(comment3 => comment3.CommentTime)) 10 select new 11 { 12 activityTitle = activity.ActivityTitle, 13 userId = comment2.CommentUserID, 14 commentId = comment2.CommentID, 15 commentContent = comment2.CommentContents, 16 17 }).FirstOrDefault(), 18 }).ToList(); var test2 = from commentlist in test 20 select new 21 { 22 activityId = commentlist.activityId, 23 activityTitle = commentlist.commentInfo.activityTitle, 24 commentId = commentlist.commentInfo.commentId, 25 }; 26 27 var test3 = (from comment in commentListContext.Comments 28 group comment by comment.CommentActivityID into c 29 select new 30 { 31 activityId = c.Key, 32 commentInfo = from comment2 in c 33 from activity in commentListContext.Activities 34 where (comment2.CommentActivityID == activity.ActivityID &&
comment2.CommentTime == c.Max(comment3 => comment3.CommentTime)) 36 select new 37 { 38 activityTitle = activity.ActivityTitle, 39 userId = comment2.CommentUserID, 40 commentId = comment2.CommentID, 41 commentContent = comment2.CommentContents, 43 }, 44 }).ToList(); 45 var test4 = from commentlist in test3 46 select new 47 { 48 activityId = commentlist.activityId, 49 //activityTitle = commentlist.commentInfo.activityTitle, 50 //commentId = commentlist.commentInfo.commentId, 51 }; 52 var test5 = from commentlist in test3 53 select new 54 { 55 activityId = commentlist.activityId, 56 commentInfo = from c in commentlist.commentInfo 57 select c, 58 };
var test = (from comment in commentListContext.Comments
group comment by comment.CommentActivityID into c
select new
{
activityId = c.Key,
commentInfo = (from comment2 in c
from activity in commentListContext.Activities
where (comment2.CommentActivityID == activity.ActivityID &&
comment2.CommentTime == c.Max(comment3 => comment3.CommentTime))
select new
{
activityTitle = activity.ActivityTitle,
userId = comment2.CommentUserID,
commentId = comment2.CommentID,
commentContent = comment2.CommentContents,
}).FirstOrDefault(),
}).ToList();
var test2 = from commentlist in test
select new
{
activityId = commentlist.activityId,
activityTitle = commentlist.commentInfo.activityTitle,
commentId = commentlist.commentInfo.commentId,
};
var test3 = (from comment in commentListContext.Comments
group comment by comment.CommentActivityID into c
select new
{
activityId = c.Key,
commentInfo = from comment2 in c
from activity in commentListContext.Activities
where (comment2.CommentActivityID == activity.ActivityID &&
comment2.CommentTime == c.Max(comment3 => comment3.CommentTime))
select new
{
activityTitle = activity.ActivityTitle,
userId = comment2.CommentUserID,
commentId = comment2.CommentID,
commentContent = comment2.CommentContents,
},
}).ToList();
var test4 = from commentlist in test3
select new
{
activityId = commentlist.activityId,
activityTitle = commentlist.commentInfo.activityTitle,//这里是错的
commentId = commentlist.commentInfo.commentId,//这里是错的
};
var test5 = from commentlist in test3
select new
{
activityId = commentlist.activityId,
commentInfo = from c in commentlist.commentInfo
select c,
};
test3没有用FirstOrDefault(),在test4那里就不能像test2那样的写法了，只能写成test5那样，请问下，这是什么原因啊？是什么原理呢？谢谢各位了。
最佳答案
0
activityTitle = commentlist.commentInfo ? null:commentlist.commentInfo.activityTitle,//这里是错的
commentId = commentlist.commentInfo ? null:commentlist.commentInfo.commentId,//这里是错的
收获园豆：10
test3没有用FirstOrDefault(),在test4，commentlist.commentInfo.activityTitle就没有这样的属性的
@wenbolwm: 你是说 test4 中的 commentlist 没有 commentInfo 属性吗？
@Launcher: 没有commentInfo.后面的属性了
@wenbolwm: 在 test4 中你的 commentlist 的 commentInfo 是 IQueryable<dynamic type> 类型的，
activityTitle = commentlist.commentInfo.FirstOrDefault() == null ?
null : commentlist.commentInfo.FirstOrDefault().activityTitle;
@Launcher: 这样可以点出属性了，能告诉下他们有什么区别？为什么这样？原理是什么吗？谢谢了
@wenbolwm: 楼下已经告诉你了，你的 test3 中的是这样的： commentInfo = from comment2 in c
这里的 commentInfo 表示一个 IQueryable<dynamic type> 类型，而不是 dynamic type 类型。
其他回答(2)
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test3没有用FirstOrDefault(),那么test3中的commentInfo是一个实体的List集合.
在Test1中commentInfo是一个实体.
//activityTitle = commentlist.commentInfo.activityTitle,
这个commentlist.commentInfo是一个集合
是不是可以认为，实体后面可以点出属性来，集合就不行了
@wenbolwm: 你这是乱理解。假设：
class CommentInfo {public string ActivityTitle{get;set;}};
List<CommentInfo> list = new List<CommentInfo>();
list.ActivityTitle; // 这能编译通过吗？
list[0].ActivityTitle; // 这能编译通过吗？
@Launcher: list[0].ActivityTitle 这个能编译通过, 理解了，谢谢你
0
test3没有用FirstOrDefault(),那么test3中的commentInfo是一个实体的List集合.
在Test1中commentInfo是一个实体.
