实体框架和lambda表达式树（深度空合并）

Question

var articles = context.Articles.Where(a => a.Id != articleId)
.OrderBy(p => p.Categories.OrderBy(q => q.Name).FirstOrDefault().Name).ToList();

i get message of possible NullReferenceException which is correct.

So I make

var  articles = context.Articles.Where(a => a.Id != articleId)
                               .OrderBy(p =>
                                   (p.Categories.OrderBy(q => q.Name).FirstOrDefault() != null
                                    ? p.Categories.OrderBy(q => q.Name).FirstOrDefault().Name
                                    : null))
                               .Skip(page * pageSize)
                                  .Take(pageSize)
                                  .ToList();

which works but statement is calling two times and can be slow so i try to make

var articles = context.Articles.Where(a => a.Id != articleId)
             .OrderBy(p =>
             {
                 var firstOrDefault = p.Categories.OrderBy(q => q.Name).FirstOrDefault();
                 return firstOrDefault != null ? firstOrDefault.Name : null;
             }).ToList();

but i get

lambda expression with a statement body can not be converted to an expression tree.

What can i do? Ss first example correct even if i call two times p.Categories.OrderBy(q => q.Name).FirstOrDefault().

I am thinking that this can be slow. I have 200k rows in database.

Answer 1

i get message of possible NullReferenceException which is correct.

It isn't clear which system is producing this message. Resharper?

Anyway, in this case, if this really is LINQ to Entities, the warning is spurious. LINQ to Entities performs automatic 'deep null coalescing' in many cases, and this is one such case.

In your original query:

var articles = context.Articles
                      .Where(a => a.Id != articleId)
                      .OrderBy(p => p.Categories
                                     .OrderBy(q => q.Name)
                                     .FirstOrDefault()
                                     .Name)
                       .ToList();

...there will be no NullReferenceException if an article has no category associated with it. Instead, the ordering value will be taken as null for such articles (meaning articles with no categories at all will turn up first), which it appears is precisely what you want. So no extra effort is required from your side!

Do note that with other LINQ providers (such as LINQ to Objects), the behaviour can be quite different, and .FirstOrDefault().XXX is indeed a risky expression.

On another note, don't prematurely optimize. If you already have a working solution, benchmark it. If it's too slow, investigate why - in this case, the clues are in the generated SQL. The LINQ to Entities query optimizer can often be smarter than you think. :)

Answer 2

Your second example will indeed produce much more complex SQL query but it is up to database if this query will really be slower. Anyway you can simply change your first query little bit and it should work as expected without that warning:

var articles = context.Articles
                      .Where(a => a.Id != articleId)
                      .OrderBy(p => p.Categories
                                     .OrderBy(q => q.Name)
                                     .Select(q => q.Name)
                                     .FirstOrDefault())
                      .ToList();

The issue in your query is selecting Name after you call FirstOrDefault so if you project the result prior to call FirstOrDefault it should not produce the warning but there will be additional subselect in resulting SQL.

Btw. @Ani's answer is correct.

Answer 3

这似乎是一个重复的答案： “带有语句体的lambda表达式无法转换为表达式树”看起来您不能使用花括号表示的代码块。