使用ForEach-Object -Parallel时自定义函数无法识别的问题

环境信息 #

Name                           Value
----                           -----
PSVersion                      7.3.3
PSEdition                      Core
GitCommitId                    7.3.3
OS                             Microsoft Windows 10.0.19044
Platform                       Win32NT
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0…}
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1
WSManStackVersion              3.0

正常执行的代码与结果 #

在普通ForEach-Object块中调用脚本顶层定义的CustomPrint函数可以正常运行：

function CustomPrint {
    param($Item)
    Write-Output $Item.Name
}
Get-ChildItem "." -Recurse -Filter "*.txt" | ForEach-Object { &CustomPrint -Item $_ }

输出：

test1.txt
test2.txt
test3.txt

添加-Parallel参数后的错误情况 #

当使用-Parallel参数后，执行脚本出现函数未识别的错误：

function CustomPrint {
    param($Item)
    Write-Output $Item.Name
}
Get-ChildItem "." -Recurse -Filter "*.txt" | ForEach-Object -Parallel { &CustomPrint -Item $_ }

错误信息：

&: The term 'CustomPrint' is not recognized as a name of a cmdlet, function, script file, or executable program.
Check the spelling of the name, or if a path was included, verify that the path is correct and try again.
&: The term 'CustomPrint' is not recognized as a name of a cmdlet, function, script file, or executable program.
Check the spelling of the name, or if a path was included, verify that the path is correct and try again.
&: The term 'CustomPrint' is not recognized as a name of a cmdlet, function, script file, or executable program.
Check the spelling of the name, or if a path was included, verify that the path is correct and try again.

解答 #

这是预期行为，原因在于：

• ForEach-Object -Parallel会为每个并行任务启动独立的PowerShell运行空间，这些运行空间默认不会继承当前会话中的自定义函数、变量等元素。普通ForEach-Object是在当前运行空间执行，所以能直接访问顶层定义的函数。

解决方法 #

有几种方式可以让并行脚本块访问自定义函数：

• 使用$using:作用域传递函数
  把函数作为变量传递到并行块中，再调用：

function CustomPrint {
    param($Item)
    Write-Output $Item.Name
}
Get-ChildItem "." -Recurse -Filter "*.txt" | ForEach-Object -Parallel {
    $customPrint = $using:CustomPrint
    &$customPrint -Item $_
}

• 在并行脚本块内部定义函数
  如果函数逻辑简单，可以直接把函数定义放在并行块里：

Get-ChildItem "." -Recurse -Filter "*.txt" | ForEach-Object -Parallel {
    function CustomPrint {
        param($Item)
        Write-Output $Item.Name
    }
    &CustomPrint -Item $_
}

• 使用-ArgumentList传递函数
  通过参数列表把函数传入并行块：

function CustomPrint {
    param($Item)
    Write-Output $Item.Name
}
Get-ChildItem "." -Recurse -Filter "*.txt" | ForEach-Object -Parallel {
    $func, $item = $args
    &$func -Item $item
} -ArgumentList $CustomPrint, $_

内容的提问来源于stack exchange，提问作者igg