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c# - Linq：选择有和没有 ToList()

问题描述

谁能解释这种行为？

此代码工作：

Dictionary<string, int> fullPathTabAssociation = new Dictionary<string, int>();

    //bla bla.. 
    //here fullPathTabAssociation is populated
    ////bla bla.. 

var newValues = fullPathTabAssociation
  .Where(x => x.Value > index)
  .Select(x => new KeyValuePair<string, int>(x.Key, x.Value - 1))
  .ToList();

fullPathTabAssociation.Clear();

/*now newValues is populated with correct values*/

此代码不起作用

Dictionary<string, int> fullPathTabAssociation = new Dictionary<string, int>();

    //bla bla.. 
    //here fullPathTabAssociation is populated
    ////bla bla.. 

var newValues = fullPathTabAssociation
  .Where(x => x.Value > index)
  .Select(x => new KeyValuePair<string, int>(x.Key, x.Value - 1))

fullPathTabAssociation.Clear();

 /*now newValues is empty*/

选择函数似乎返回一个新IEnumerable的，在这两种情况下，在调试之前fullPathTabAssociation.Clear()，值都是正确的newValues并且不同于fullPathTabAssociation. 特别是我不明白最后一种情况会发生什么

标签： c#linqpointersfunction-pointers

解决方案

Linq 是懒惰的；它尽可能地推迟它的工作（直到foreach或某种具体化）。

在第一个摘录中，您在.ToList()此处实现查询，Linq 必须执行并提供List<T>集合：

var newValues = fullPathTabAssociation
  .Where(x => x.Value > index)
  .Select(x => new KeyValuePair<string, int>(x.Key, x.Value - 1))
  .ToList(); // <- Materizataion an actual List<T> required; Linq executes 

// Since we have newValues collection (not query) it doesn't depend on аullPathTabAssociation
fullPathTabAssociation.Clear();

在第二个摘录中，Linq不需要做任何事情：

// Just a query, no materialization
var newValues = fullPathTabAssociation
  .Where(x => x.Value > index)
  .Select(x => new KeyValuePair<string, int>(x.Key, x.Value - 1));

fullPathTabAssociation.Clear();

...

// Only here (on foreach or materialization) Linq has to execute the query; 
// and it takes fullPathTabAssociation (which is empty) 
foreach (var item in newValues) {
  ...
}

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