差之间PowerShell和C#枚举集合
问题描述:
这里的时候是在C#一个简单的场景:差之间PowerShell和C#枚举集合
var intList = new List<int>();
intList.Add(4);
intList.Add(7);
intList.Add(2);
intList.Add(9);
intList.Add(6);
foreach (var num in intList)
{
if (num == 9)
{
intList.Remove(num);
Console.WriteLine("Removed item: " + num);
}
Console.WriteLine("Number is: " + num);
}
这引发InvalidOperationException
因为我修改集合,而枚举它。
现在考虑类似的PowerShell代码:
$intList = 4, 7, 2, 9, 6
foreach ($num in $intList)
{
if ($num -eq 9)
{
$intList = @($intList | Where-Object {$_ -ne $num})
Write-Host "Removed item: " $num
}
Write-Host "Number is: " $num
}
Write-Host $intList
这个脚本实际上是从列表中删除9号!没有例外抛出。
现在,我知道C#示例使用List对象,而PowerShell示例使用数组,但PowerShell如何枚举将在循环期间被修改的集合?
答
答案已经被@Sean给,我只是提供代码显示原始集合在foreach
期间没有更改:它通过原始集合列举,因此没有矛盾。
# original array
$intList = 4, 7, 2, 9, 6
# make another reference to be used for watching of $intList replacement
$anotherReferenceToOriginal = $intList
# prove this: it is not a copy, it is a reference to the original:
# change [0] in the original, see the change through its reference
$intList[0] = 5
$anotherReferenceToOriginal[0] # it is 5, not 4
# foreach internally calls GetEnumerator() on $intList once;
# this enumerator is for the array, not the variable $intList
foreach ($num in $intList)
{
[object]::ReferenceEquals($anotherReferenceToOriginal, $intList)
if ($num -eq 9)
{
# this creates another array and $intList after assignment just contains
# a reference to this new array, the original is not changed, see later;
# this does not affect the loop enumerator and its collection
$intList = @($intList | Where-Object {$_ -ne $num})
Write-Host "Removed item: " $num
[object]::ReferenceEquals($anotherReferenceToOriginal, $intList)
}
Write-Host "Number is: " $num
}
# this is a new array, not the original
Write-Host $intList
# this is the original, it is not changed
Write-Host $anotherReferenceToOriginal
输出:
5
True
Number is: 5
True
Number is: 7
True
Number is: 2
True
Removed item: 9
False
Number is: 9
False
Number is: 6
5 7 2 6
5 7 2 9 6
我们可以看到,当我们 “删除项目” 是$intList
改变。它只意味着此变量现在包含对新数组的引用,它是已更改的变量,而不是数组。循环继续枚举未改变的原始数组,并且$anotherReferenceToOriginal
仍然包含对它的引用。
答
foreach构造函数计算完成的列表并将结果存储在临时变量中,然后开始迭代它。当你做这个实际移除时,你正在更新$ intList来引用一个新列表。在实际做这样的事情在引擎盖下换句话说:
$intList = 4, 7, 2, 9, 6
$tempList=$intList
foreach ($num in $tempList)
{
if ($num -eq 9)
{
$intList = @($intList | Where-Object {$_ -ne $num})
Write-Host "Removed item: " $num
}
Write-Host "Number is: " $num
}
Write-Host $intList
您的来电:
$intList = @($intList | Where-Object {$_ -ne $num})
实际创建与删除的值完全新的列表。
如果您更改删除逻辑以删除列表中的最后一项(6),那么我认为即使您认为它因为临时副本而被删除,仍会打印它。
答
这里的问题是,你没有比较等效的代码示例。在Powershell示例中,您正在创建一个新列表,而不是像在C#示例中那样修改列表。这里是一个样本,其在功能上更接近原始C#一个
$intList = new-object System.Collections.ArrayList
$intList.Add(4)
$intList.Add(7)
$intList.Add(2)
$intList.Add(9)
$intList.Add(6)
foreach ($num in $intList) {
if ($num -eq 9) {
$intList.Remove($num)
Write-Host "Removed item: " $num
}
Write-Host "Number is: " $num
}
Write-Host $intList
并运行时,它产生同样的错误
Number is: 4
Number is: 7
Number is: 2
Removed item: 9
Number is: 9
An error occurred while enumerating through a collection: Collection was modifi
ed; enumeration operation may not execute..
At C:\Users\jaredpar\temp\test.ps1:10 char:8
+ foreach <<<< ($num in $intList)
+ CategoryInfo : InvalidOperation: (System.Collecti...numeratorSi
mple:ArrayListEnumeratorSimple) [], RuntimeException
+ FullyQualifiedErrorId : BadEnumeration
4 7 2 6
JaredPar的答案指出foreach构造不会创建临时列表副本。 – Greg 2010-11-23 19:30:55
@Greg:不,JaredPar使用ArrayList。 Powershell代码中没有任何内容表明列表是List 或ArrayList,这是一个实现细节。 – Sean 2010-11-23 19:59:24