IEnumerable.Select和SelectMany的区别

IEnumerable<T>在Windows Phone 7的程序上很常用，它允许开发人员定义foreach语句功能的实现并支持非泛型方法的简单迭代，下面主要分析一下 IEnumerable<T>.Select和IEnumerable<T>.SelectMany这两个方法的区别。

IEnumerable<T>.Select  将序列中的每个元素投影到新表中。

IEnumerable<T>.SelectMany 将序列的每个元素投影到 IEnumerable<T> 并将结果序列合并为一个序列。

SelectMany 方法枚举输入序列，使用转换函数将每个元素映射到 IEnumerable<T>，然后枚举并生成每个这种 IEnumerable<T> 对象的元素。 也就是说，对于 source 的每个元素，selector 被调用，返回一个值的序列。 然后 SelectMany将集合的此二维集合合并为一维 IEnumerable<T> 并将其返回。

下面一个小例子用IEnumerable<T>.Select和IEnumerable<T>.SelectMany实现同样的功能，看看两者的区别。

<Grid x:Name="ContentPanel" Grid.Row="1" Margin="12,0,12,0">            <ListBox  Name="Output"                          HorizontalAlignment="Stretch"                            VerticalAlignment="Stretch"                           Margin="10" />            <ListBox  Name="Output2"                          HorizontalAlignment="Stretch"                            VerticalAlignment="Stretch"                           Margin="200,10,10,10" /></Grid>

cs页面

namespace LinqApplication{    public partial class MainPage : PhoneApplicationPage    {        List<Book> Books;//List<T>继承了IEnumerable<T> 接口        public MainPage()        {            InitializeComponent();            CreateBooks();            //IEnumerable<Book>.Select 将序列中的Authors元素投影到新表中。            IEnumerable<List<Author>> EnumerableOfListOfAuthors = Books.Select(book => book.Authors);            foreach (var listOfAuthors in EnumerableOfListOfAuthors)            {                 foreach (Author auth in listOfAuthors)                {                    Output.Items.Add(auth.Name); //添加到ListBox里面                }            }            //IEnumerable<Book>.SelectMany  将序列的每个元素投影到 IEnumerable<T> 并将结果序列合并为一个序列。            IEnumerable<Author> authors = Books.SelectMany(book => book.Authors);            foreach (Author auth in authors)             {                 Output2.Items.Add(auth.Name);            }         }        private void CreateBooks()         {             Books = new List<Book>();             Author auth1 = new Author() { Name = "张三", Age = 32 };            Author auth2 = new Author() { Name = "李四", Age = 30 };            Author auth3 = new Author() { Name = "加菲猫", Age = 31 };            List<Author> authors = new List<Author>() { auth1, auth2, auth3 };            Book newBook = new Book() { Authors = authors, NumPages = 500, Title = "Programming C#" };            Books.Add(newBook); auth1 = new Author() { Name = "刘德华", Age = 42 };            authors = new List<Author>() { auth1 };            newBook = new Book() { Authors = authors, NumPages = 350, Title = "Book 2" };            Books.Add(newBook); auth1 = new Author() { Name = "周杰伦", Age = 32 };            auth2 = new Author() { Name = "林志玲", Age = 32 };             authors = new List<Author>() { auth1, auth2 };            newBook = new Book() { Authors = authors, NumPages = 375, Title = "Programming with WP7" };             Books.Add(newBook);        }     }}

Author.cs类

namespace LinqApplication{    public class Author    {        public string Name;         public int Age;     }}

Book.cs类

namespace LinqApplication{    public class Book    {        public String Title { get; set; }         public List<Author> Authors { get; set; }         public int NumPages { get; set; }     }}