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，請使用 SelectMany()，或者你正在建立序列序列，但是你希望將結果作為一個長序列。

在 LINQ 查詢語法中：

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

如果你有一個集合集合，並希望能夠同時處理來自父集合和子集合的資料，那麼也可以使用 SelectMany。

我們來定義簡單的類

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; }
}

我們假設我們有以下收集。

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"
            }
        }
    }
};

現在我們要選擇評論 Content 以及與此評論相關的 BlogPost 的 Id。為了做到這一點，我們可以使用適當的 SelectMany 過載。

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

我們的 commentsWithIds 看起來像這樣

{
    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"
}