SelectMany 展平一系列序列

var sequenceOfSequences = new [] { new [] { 1, 2, 3 }, new [] { 4, 5 }, new [] { 6 } };
var sequence = sequenceOfSequences.SelectMany(x => x);
// returns { 1, 2, 3, 4, 5, 6 }

如果有，请使用 SelectMany()，或者你正在创建序列序列，但是你希望将结果作为一个长序列。

在 LINQ 查询语法中：

var sequence = from subSequence in sequenceOfSequences
               from item in subSequence
               select item;

如果你有一个集合集合，并希望能够同时处理来自父集合和子集合的数据，那么也可以使用 SelectMany。

我们来定义简单的类

public class BlogPost
{
    public int Id { get; set; }
    public string Content { get; set; }
    public List<Comment> Comments { get; set; }
}

public class Comment
{
    public int Id { get; set; }
    public string Content { get; set; }
}

我们假设我们有以下收集。

List<BlogPost> posts = new List<BlogPost>()
{
    new BlogPost()
    {
        Id = 1,
        Comments = new List<Comment>()
        {
            new Comment()
            {
                Id = 1,
                Content = "It's really great!",
            },
            new Comment()
            {
                Id = 2,
                Content = "Cool post!"
            }
        }
    },
    new BlogPost()
    {
        Id = 2,
        Comments = new List<Comment>()
        {
            new Comment()
            {
                Id = 3,
                Content = "I don't think you're right",
            },
            new Comment()
            {
                Id = 4,
                Content = "This post is a complete nonsense"
            }
        }
    }
};

现在我们要选择评论 Content 以及与此评论相关的 BlogPost 的 Id。为了做到这一点，我们可以使用适当的 SelectMany 重载。

var commentsWithIds = posts.SelectMany(p => p.Comments, (post, comment) => new { PostId = post.Id, CommentContent = comment.Content });

我们的 commentsWithIds 看起来像这样

{
    PostId = 1,
    CommentContent = "It's really great!"
},
{
    PostId = 1,
    CommentContent = "Cool post!"
},
{
    PostId = 2,
    CommentContent = "I don't think you're right"
},
{
    PostId = 2,
    CommentContent = "This post is a complete nonsense"
}