如果您使用成员身份并且不想将aspnet_Users包含在dbml中，则用于显示用户名：

...
LastPostUserId = posts.OrderByDescending(p=>p.PostId).Take(1).Select(p=> Membership.GetUser(p.UserId))
...

使发布的示例更好一点的另一项更改是在posts变量中添加orderbydescending：然后，您可以从select子句中删除重复4次的OrderByDescending：

from forum in Forums
let posts = ForumPosts.Where(p => p.ForumThreads.ForumId.Equals(forum.ForumId)).OrderByDescending(p=>p.PostId)
select new
{
    Forum = forum.Title,
    Description = forum.Description,
    Topics = forum.ForumThreads.Count(),
    Posts = posts.Count(),
    LastPostId = posts.Take(1).Select(p=>p.PostId),
    LastPostThreadId = posts.Take(1).Select(p=>p.ThreadId),
    LastPostUserId = posts.Take(1).Select(p=>p.UserId),
    LastPostTime = posts.Take(1).Select(p=>p.CreateDate)
}

甚至更清洁：

from forum in Forums
let posts = ForumPosts.Where(p => p.ForumThreads.ForumId.Equals(forum.ForumId))
let lastPost = posts.OrderByDescending(p=>p.PostId).Take(1)
select new
{
    Forum = forum.Title,
    Description = forum.Description,
    Topics = forum.ForumThreads.Count(),
    Posts = posts.Count(),
    LastPostId = lastPost.PostId,
    LastPostThreadId = lastPost.ThreadId,
    LastPostUserId = lastPost.UserId,
    LastPostUserName = Membership.GetUser(lastPost.UserId),
    LastPostTime = lastPost.CreateDate
}

如果没有最新帖子，请测试此代码，我认为如果Take（1）为null，则可能会引发错误。