试图了解Dapper Multi Mapping

我一直在阅读Dapper的Multi Mapping文档，但仍然感到困惑。

这是Dapper文档中的示例

class Post
{
    public int Id { get; set; }
    public string Title { get; set; }
    public string Content { get; set; }
    public User Owner { get; set; }
}

class User
{
    public int Id { get; set; }
    public string Name { get; set; }
}

var sql = 
@"select * from #Posts p 
left join #Users u on u.Id = p.OwnerId 
Order by p.Id";

var data = connection.Query<Post, User, Post>(sql, (post, user) => { post.Owner = user; return post;});
var post = data.First();

令我困惑的是变量post和user来自哪里？我看到这一行......

connection.Query<Post, User, Post>

Post和User是模型，我明白..

(post, user) => { post.Owner = user; return post;}

是一个定义映射的函数，但post和user在哪里实例化？