如何正确使用List的ForEach()语句? [英] How do I properly use ForEach() statement of List?

问题描述

我对List的ForEach方法语法中做错的事情感到困惑吗?

PS D:\ntt> $nicInfo.IpConfigurations.Count
2
PS D:\ntt> $nicInfo.IpConfigurations[0]

PrivateIpAddressVersion Name      Primary PrivateIpAddress PrivateIpAllocationMethod Subnet Name PublicIpAddress Name ProvisioningState
----------------------- ----      ------- ---------------- ------------------------- ----------- -------------------- -----------------
IPv4                    ipconfig1 True    10.233.0.4       Dynamic                                                    Succeeded


PS D:\ntt> $nicInfo.IpConfigurations.GetType()

IsPublic IsSerial Name                                     BaseType
-------- -------- ----                                     --------
True     True     List`1                                   System.Object


PS D:\ntt> $nicInfo.IpConfigurations.ForEach({$_})
PS D:\ntt>

解决方案

问题是 PetSerAl 的提示，以提供评论中的关键指针. >

解决方法:

• 如果您传递给.ForEach()的脚本块不需要产生任何输出，则所需的只是在脚本块中使用$args[0]代替$_您仍可以选择使用以下其他解决方法之一，以避免造成混淆.

• 如果需要输出 ，最简单的解决方案是先将List<T>实例转换为 array ，首先使用.ToArray() >，.ForEach()在其上可正常工作；一个简化的例子:

  $list = [System.Collections.Generic.List[object]] ('foo', 'bar')
  $list.ToArray().ForEach({ $_ + '!' }) # Note the .ToArray() call.
  

  上面的代码产生了'foo!', 'bar!'，与预期的一样.

  • 或者，您可以使用:

    • 一个foreach循环来处理列表项，这意味着您必须选择一个迭代变量名称，并在循环体中引用该变量而不是$_；例如:
      foreach ($itm in $list) { $itm + '!' }
    管道中的
    • 或ForEach-Object(速度较慢，但​​不需要更改脚本块)，如；例如:
      $list | ForEach-Object { $_ + '!' }

I'm confused what I'm doing wrong in ForEach method syntax of List?

PS D:\ntt> $nicInfo.IpConfigurations.Count
2
PS D:\ntt> $nicInfo.IpConfigurations[0]

PrivateIpAddressVersion Name      Primary PrivateIpAddress PrivateIpAllocationMethod Subnet Name PublicIpAddress Name ProvisioningState
----------------------- ----      ------- ---------------- ------------------------- ----------- -------------------- -----------------
IPv4                    ipconfig1 True    10.233.0.4       Dynamic                                                    Succeeded


PS D:\ntt> $nicInfo.IpConfigurations.GetType()

IsPublic IsSerial Name                                     BaseType
-------- -------- ----                                     --------
True     True     List`1                                   System.Object


PS D:\ntt> $nicInfo.IpConfigurations.ForEach({$_})
PS D:\ntt>

解决方案

The problem is that PowerShell's own .ForEach() collection method is preempted by the List<T> type's own .ForEach() method in this case:

• PowerShell's own .ForEach({ ... }):

  • defines $_ as the input object at hand for the script-block argument ({ ... })
  • passes any output produced inside the script block through (to PowerShell's success output stream).

• By contrast, List<T>'s .ForEach({ ... }) converts the script block to an Action<T> delegate, which has the following implications:

  • The delegate doesn't know about $_ inside the script block and instead receives a single argument that must be accessed as $args[0].

  • Output from the script block is ignored, because an Action<T> delegate by definition has no return value.

    • While you can produce host (console) output with Write-Host from within the script block, such output cannot be used programmatically, because it bypasses PowerShell's output streams and can therefore neither be captured nor redirected.

^{Tip of the hat to PetSerAl for providing the crucial pointers in comments.}

Workarounds:

• If the script block you pass to .ForEach() need not produce any output, all that's needed is to use $args[0] in lieu of $_ in your script block, though you may still opt to use one of the other workarounds below in order to avoid confusion.

• If output is needed, the simplest solution is to convert the List<T> instance to an array with .ToArray() first, on which .ForEach() works as expected; a simplified example:

  $list = [System.Collections.Generic.List[object]] ('foo', 'bar')
  $list.ToArray().ForEach({ $_ + '!' }) # Note the .ToArray() call.
  

  The above produces 'foo!', 'bar!', as expected.

  • Alternatively, you may use:

    • a foreach loop to process the list items, which means you must pick an iteration variable name and refer to that instead of $_ in the loop body; e.g.:
      foreach ($itm in $list) { $itm + '!' }
    • or ForEach-Object in a pipeline (slower, but doesn't require changing the script block), as shown in No Refunds No Returns' answer; e.g.:
      $list | ForEach-Object { $_ + '!' }

这篇关于如何正确使用List的ForEach()语句?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！