Windows Phone 7 IEnumerable<T>.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 newnewBook = 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 };
newnewBook = 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 };
newnewBook = 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; }

}
}