从LINQ查询语法转换为方法语法

Question

I have the following join:

var simpleJoin = from b in books
                 join p in publishers on b.PublisherName equals p.Name
                 where p.Id == 1
                 select b;

What is the equivalent using the method syntax? I'm getting tripped up by the filter:

simpleJoin = books.Join(publishers, p => p.PublisherName, b => b.Name, (b, p) => b).Where(*can't access publishers here*)

Can I not use books as my source collection? I'm wondering how we could manage filtering if we have multiple joins.

Answer 1

You'll need to include both b and p in the resultSelector . For example, using an anonymous-typed object:

simpleJoin = books.Join(publishers, p => p.PublisherName, b => b.Name, 
        (b, p) => new { b = b, p = p })
    .Where(result => result.p.Id == 1)
    .Select(result => result.b);

Answer 2

You can filter the publishers list prior to joining it to books :

var simpleJoin = books.Join(publishers.Where(p => p.Id == 1),
                             b => b.PublisherName, p => p.Name, (b, p) => b);

Answer 3

仅作为示例-您也可以不使用join子句：

books.Where(b => publishers.Exists(p => p.Name == b.PublisherName && p.Id == 1));