c# - 加入/ LINQ和Lambda的位置(Join/Where with LINQ and Lambda)

Question

I'm having trouble with a query written in LINQ and Lambda.

(我在使用LINQ和Lambda编写的查询时遇到问题。)

So far, I'm getting a lot of errors here's my code:

(到目前为止，我的代码中出现了很多错误：)

int id = 1;
var query = database.Posts.Join(database.Post_Metas,
                                post => database.Posts.Where(x => x.ID == id),
                                meta => database.Post_Metas.Where(x => x.Post_ID == id),
                                (post, meta) => new { Post = post, Meta = meta });

I'm new to using LINQ, so I'm not sure if this query is correct.

(我是新手使用LINQ，所以我不确定这个查询是否正确。)

  ask by David translate from so

Answer 1

I find that if you're familiar with SQL syntax, using the LINQ query syntax is much clearer, more natural, and makes it easier to spot errors:

(我发现，如果您熟悉SQL语法，那么使用LINQ查询语法会更清晰，更自然，并且更容易发现错误：)

var id = 1;
var query =
   from post in database.Posts
   join meta in database.Post_Metas on post.ID equals meta.Post_ID
   where post.ID == id
   select new { Post = post, Meta = meta };

If you're really stuck on using lambdas though, your syntax is quite a bit off.

(如果你真的坚持使用lambdas，你的语法有点偏。)

Here's the same query, using the LINQ extension methods:

(这是使用LINQ扩展方法的相同查询：)

var id = 1;
var query = database.Posts    // your starting point - table in the "from" statement
   .Join(database.Post_Metas, // the source table of the inner join
      post => post.ID,        // Select the primary key (the first part of the "on" clause in an sql "join" statement)
      meta => meta.Post_ID,   // Select the foreign key (the second part of the "on" clause)
      (post, meta) => new { Post = post, Meta = meta }) // selection
   .Where(postAndMeta => postAndMeta.Post.ID == id);    // where statement