SelectMany Linq

Question

任何人都可以解释这个SelectMany语法

List<string> animals = new List<string>() { "cat", "dog", "donkey" };
List<int> number = new List<int>() { 10, 20 };
var mix=number.SelectMany(num => animals, (n, a) => new { n, a });

我不熟悉这种语法，因为我习惯于将它用于展平（以展平列表中的列表）

 public class x
 {
     public List<z> Zs{get; set;}
     public int Y { get; set; }
 }
 public class z
 {
     public int Y { get; set; }
 }
    List<x> Xs = new List<x>();
    Xs.Add(new x() { Zs = new List<z>() { new z() { Y = 15 } } });
    Xs.Add(new x() { Zs = new List<z>() { new z() { Y = 17 } } });
    Xs.Add(new x() { Zs = new List<z>() { new z() { Y = 19 } } });
    Xs.Add(new x() { Zs = new List<z>() { new z() { Y = 25 } } });
    Xs.Add(new x() { Zs = new List<z>() { new z() { Y = 50 } } });

Xs.SelectMany(x => x.Zs).Sum(z => z.Y);

Answer 1

检查来自Microsoft here

的参考来源

以下是正在使用的SelectMany重载

        public static IEnumerable<TResult> SelectMany<TSource, TCollection, TResult>(this IEnumerable<TSource> source, Func<TSource, IEnumerable<TCollection>> collectionSelector, Func<TSource, TCollection, TResult> resultSelector) 
        {
            if (source == null) throw Error.ArgumentNull("source");
            if (collectionSelector == null) throw Error.ArgumentNull("collectionSelector");
            if (resultSelector == null) throw Error.ArgumentNull("resultSelector");
            return SelectManyIterator<TSource, TCollection, TResult>(source, collectionSelector, resultSelector);
        }

        static IEnumerable<TResult> SelectManyIterator<TSource, TCollection, TResult>(IEnumerable<TSource> source, Func<TSource, IEnumerable<TCollection>> collectionSelector, Func<TSource, TCollection, TResult> resultSelector) 
        {
            foreach (TSource element in source) 
            {
                foreach (TCollection subElement in collectionSelector(element)) 
                {
                    yield return resultSelector(element, subElement);
                }
            }
        }

工作原理（这也是数据展平的形式）

1. 类似于同心for / foreach循环，遍历两个集合并创建组合列表，其复杂度为O（N ^ 2）
2. 首先，我们提供号码列表IEnumerable<TSource>
3. 添加一个Func，每个number我们提供animal列表

4. 最后只是将结果作为组合的匿名集合，每个数字都添加一个动物，结果就像

     n    a
   
     10   cat 
     10   dog 
     10   donkey 
     20   cat 
     20   dog 
     20   donkey