How do I properly use ForEach() statement of List?

问题:

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>

回答1:

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 { $_ + '!' }

回答2:

Are you trying to do something with each item in the collection? Do you want to do something like this:

$nicInfo.IpConfigurations | ForEach-Object {
  $ipConfiguration = $_
  write-Output $ipConfiguration
  # do more stuff with this $ipConfiguration
}

回答3:

Just for your reference, you can use this code for list.ForEach().

$nicInfo.IpConfigurations.ForEach({write-host $args[0].ToString()})

And I test it myself, it works. Sample code as below:

$s=New-Object System.Collections.Generic.List[string]
$s.Add("hello_1")
$s.Add("hello_2")
$s.Add("hello_3")
$s.ForEach({write-host $args[0].ToString()})

Test result as below:

As well as I found this similar issue, @PetSerAl explained very well there.