Question

任何人都可以解释这种行为吗？

此代码有效：

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*/

此代码无效

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*/

select函数似乎返回了一个新的IEnumerable，在这两种情况下，调试都在fullPathTabAssociation.Clear()之前进行，这两个值对于newValues是正确的，并且与fullPathTabAssociation不同。特别是我不明白最后一种情况会发生什么

Answer 1

区别在于，通过使用ToList您正在执行Linq查询并创建新列表。此新创建的列表的内容与fullPathTabAssociation无关。

在第二个示例中，您仅将查询存储在newValues中。该查询尚未执行，仅在清除查询源之后才会执行。因此，查询结果为空。

Answer 2

Linq很懒；它会尽可能推迟工作（最多foreach或某种物化）。

在第一个摘录中，您使用.ToList()来实现查询，此处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();

在第二个摘录的Linq中，不必做任何事情：

// 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) {
  ...
}

Linq：选择是否带有ToList（）

2 个答案: