Select()和SelectMany()的区别

时间:2021-04-21 05:01:11

Select与SelectMany的区别

Select() 和 SelectMany() 的工作都是依据源值生成一个或多个结果值。

Select() 为每个源值生成一个结果值。因此,总体结果是一个与源集合具有相同元素数目的集合。与之相反,SelectMany() 将生成单一总体结果,其中包含来自每个源值的串联子集合。作为参数传递到 SelectMany() 的转换函数必须为每个源值返回一个可枚举值序列。然后,SelectMany() 将串联这些可枚举序列以创建一个大的序列。

string[] text ={ "Albert was here", "Burke slept late", "Connor is happy" };  

var tokens = text.Select(s => s.Split(''));

 foreach (string[] line in tokens)

      foreach (string token in line)        

      Console.Write("{0}.", token);

string[] text ={ "Albert was here", "Burke slept late", "Connor is happy" };
var tokens = text.SelectMany(s => s.Split('')); foreach (string token in tokens)   Console.Write("{0}.", token);

Select()和SelectMany()的区别

下面两个插图演示了这两个方法的操作之间的概念性区别。在每种情况下,假定选择器(转换)函数从每个源值中选择一个由花卉数据组成的数组。

下图描述 Select() 如何返回一个与源集合具有相同元素数目的集合。

Select()和SelectMany()的区别

下图描述 SelectMany() 如何将中间数组序列串联为一个最终结果值,其中包含每个中间数组中的每个值。

Select()和SelectMany()的区别

CSDN的例子:

class Bouquet
{
public List<string> Flowers { get; set; }
} static void SelectVsSelectMany()
{
List<Bouquet> bouquets = new List<Bouquet>() {
new Bouquet { Flowers = new List<string> { "sunflower", "daisy", "daffodil", "larkspur" }},
new Bouquet{ Flowers = new List<string> { "tulip", "rose", "orchid" }},
new Bouquet{ Flowers = new List<string> { "gladiolis", "lily", "snapdragon", "aster", "protea" }},
new Bouquet{ Flowers = new List<string> { "larkspur", "lilac", "iris", "dahlia" }}
}; // *********** Select ***********
IEnumerable<List<string>> query1 = bouquets.Select(bq => bq.Flowers); // ********* SelectMany *********
IEnumerable<string> query2 = bouquets.SelectMany(bq => bq.Flowers); Console.WriteLine("Results by using Select():");
// Note the extra foreach loop here.
foreach (IEnumerable<String> collection in query1)
foreach (string item in collection)
Console.WriteLine(item); Console.WriteLine("\nResults by using SelectMany():");
foreach (string item in query2)
Console.WriteLine(item); /* This code produces the following output: Results by using Select():
sunflower
daisy
daffodil
larkspur
tulip
rose
orchid
gladiolis
lily
snapdragon
aster
protea
larkspur
lilac
iris
dahlia Results by using SelectMany():
sunflower
daisy
daffodil
larkspur
tulip
rose
orchid
gladiolis
lily
snapdragon
aster
protea
larkspur
lilac
iris
dahlia
*/ }

   Select() 和 SelectMany() 的工作都是依据源值生成一个或多个结果值。 Select() 为每个源值生成一个结果值。 因此,总体结果是一个与源集合具有相同元素数目的集合。 与之相反,SelectMany() 将生成单一总体结果,其中包含来自每个源值的串联子集合。

  SelectMany()生成的是单一的总体结果。请看SelectMany()函数的实现形式:

 public static IEnumerable<TResult> SelectMany<TSource, TResult>(this IEnumerable<TSource> source,
Func<TSource, IEnumerable<TResult>> selector) {
if (source == null) throw Error.ArgumentNull("source");
if (selector == null) throw Error.ArgumentNull("selector");
return SelectManyIterator<TSource, TResult>(source, selector);
} static IEnumerable<TResult> SelectManyIterator<TSource, TResult>(IEnumerable<TSource> source,
Func<TSource, IEnumerable<TResult>> selector) {
foreach (TSource element in source) {
foreach (TResult subElement in selector(element)) {
yield return subElement;
}
}
}

在代码中的红色部分,可以看出SelectMany()的实现过程。在第一个例子中的SelectMany()。

string[] text ={ "Albert was here", "Burke slept late", "Connor is happy" }; 
            var tokens = text.SelectMany(s => s.Split(''));

text.SelectMany(s => s.Split(''));

可以用SelectManyIterator<TSource, TResult>(IEnumerable<TSource> source,Func<TSource, IEnumerable<TResult>> selector)替换,变成 SelectManyIterator(text,s=>s.Splt(' '));而在SelectManyIterator内部的代码相应的变为:

foreach (string element in text) {                            text是string[]数组

foreach (string subElement in s=>s.Split(' ')) {               s.Split(' ')则编程text元素生成的可枚举数组序列,IEnumerable<string>
                                yield return subElement;}}

以上转换可以看下图:

Select()和SelectMany()的区别

上图中selector是Func<TSource,IEnumerable<TResult>>谓语表达式,相当于委托。示例中委托的又来克看下图。
Select()和SelectMany()的区别

以上是我对Select()和SelectMany()的理解,如有错误欢迎指正:

参看CSDN的例子。网址:http://msdn.microsoft.com/zh-cn/library/bb546168.aspx#Mtps_DropDownFilterText

博主:itjeff ,网址:http://www.cnblogs.com/itjeff/p/3368627.html

itjeff

itjeff