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how to use .Foreach() method for a list in powershell? RRS feed

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    I wanna use list.Foreach() method in powershell, sample code as below:

    $s=New-Object System.Collections.Generic.List[string]
$s.Add("hello_1")
$s.Add("hello_2")
$s.Add("hello_3")

    if I use $s.foreach({$_}), then nothing output. can you tell me why?
    I also find some other usage which can work well, like $s.GetEnumerator().foreach({$_}) or 
    $s.ForEach({write-host $args[0].ToString()}).
    But now I want to know why $s.foreach({$_}) does not work? any reasons?

    Tuesday, December 18, 2018 5:59 AM

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    Look at the definition of the argument for "ForEach":

    PS D:\scripts> $s.ForEach

OverloadDefinitions
-------------------
void ForEach(System.Action[string] action)

    Notice it wants an "Action" object.

    PS D:\scripts> [System.Action[string]]

IsPublic IsSerial Name                                     BaseType
-------- -------- ----                                     --------
True     True     Action`1                                 System.MulticastDelegate

    The Action object is a multicast delegate. 

    The construct is designed to support the Linq syntax.  See the following.

    https://stackoverflow.com/questions/2834094/what-is-actionstring

    In PowerShell:

    PS D:\scripts> $s = New-Object System.Collections.Generic.List[string]
PS D:\scripts> $s.Add("hello_1")
PS D:\scripts> $s.Add("hello_2")
PS D:\scripts> $s.Add("hello_3")
PS D:\scripts> $s.ForEach({
>>         param ($x)
>>         Write-Host $x
>> })
hello_1
hello_2
hello_3
PS D:\scripts>

    \_(ツ)_/

    • Proposed as answer by eeeww Wednesday, December 19, 2018 5:18 AM
    • Marked as answer by ime11 Wednesday, December 19, 2018 5:20 AM
    Tuesday, December 18, 2018 8:50 AM

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

    Trying to use ForEach() method and confused what I'm doing wrong

    You might ask Microsoft why it is like it is.  ;-)   ... if you think it's a bug you should report it to Microsoft.

    Live long and prosper!

    (79,108,97,102|%{[char]$_})-join''

    Tuesday, December 18, 2018 7:05 AM
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    Look at the definition of the argument for "ForEach":

    PS D:\scripts> $s.ForEach

OverloadDefinitions
-------------------
void ForEach(System.Action[string] action)

    Notice it wants an "Action" object.

    PS D:\scripts> [System.Action[string]]

IsPublic IsSerial Name                                     BaseType
-------- -------- ----                                     --------
True     True     Action`1                                 System.MulticastDelegate

    The Action object is a multicast delegate. 

    The construct is designed to support the Linq syntax.  See the following.

    https://stackoverflow.com/questions/2834094/what-is-actionstring

    In PowerShell:

    PS D:\scripts> $s = New-Object System.Collections.Generic.List[string]
PS D:\scripts> $s.Add("hello_1")
PS D:\scripts> $s.Add("hello_2")
PS D:\scripts> $s.Add("hello_3")
PS D:\scripts> $s.ForEach({
>>         param ($x)
>>         Write-Host $x
>> })
hello_1
hello_2
hello_3
PS D:\scripts>

    \_(ツ)_/

    • Proposed as answer by eeeww Wednesday, December 19, 2018 5:18 AM
    • Marked as answer by ime11 Wednesday, December 19, 2018 5:20 AM
    Tuesday, December 18, 2018 8:50 AM
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    PS D:\scripts> $ints = New-Object System.Collections.Generic.List[int]
PS D:\scripts> $ints.Add(1)
PS D:\scripts> $ints.Add(2)
PS D:\scripts> $ints.Add(3)
PS D:\scripts> $ints.ForEach({
>>     param ($i)
>>     Write-Host ($i * 10)
>> })
10
20
30
PS D:\scripts>
    PS D:\scripts> $ints.FindAll({Param($i) $i -gt 1})
    2
    3
    PS D:\scripts>

    \_(ツ)_/


    • Edited by jrv Tuesday, December 18, 2018 8:57 AM
    Tuesday, December 18, 2018 8:56 AM
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    All of the Linq extensions to the PS/Net objects take a delegate.  The delegate in PS is created from a scriptblock that is passed to the extension method.

    For simple collections we can use the pipeline variable:

    PS D:\scripts> $a = 1,2,3,4,5,6
PS D:\scripts> $a.ForEach({$_})
1
2
3
4
5
6
PS D:\scripts> $a.Where({$_ -gt 3})
4
5
6

    We can also call complex code in a function

    PS D:\scripts> function foo{Write-Host $_}
PS D:\scripts> $a.ForEach($function:foo)
1
2
3
4
5
6

    \_(ツ)_/

    Tuesday, December 18, 2018 9:06 AM