如何在Foreach-Object -Parallel外部获取循环内设置的变量值

当使用Foreach-Object -Parallel结合While循环时，无法在循环外部获取内部设置的变量值。比如以下简化脚本中，最终Write-Host输出的$limitReached无值，但预期应为TRUE。

原脚本示例 #

$retries = 2
$secondsDelay = 2
$a = "house"

1..2 | ForEach-Object -Parallel {
    $retryCount = 0
    $ready = $false

    while ($ready -ne $true)
    {      
        if ($a -eq "hello")
        {
            Write-Output ("a is equal ""hello""".)
            $ready = $true                    
        }
        else
        {
            Write-Output ("a is not equal ""hello""".)

            if ($retryCount -ge $using:retries)
            {
                Write-Output ("a is STILL not equal ""hello""".)
                $global:limitReached = $true
                Write-Host "Checking - limitReached value is [$limitReached]"
                return
            }
            else
            {
                Write-Output ("let's give more runs.")`n
                Start-Sleep $using:secondsDelay
                $retryCount++
            }                    
        }
    }        
}

Write-Host "I need this value - limitReached here is [$global:limitReached]"

结果对比 #

• 预期结果：I need this value - limitReached here is [TRUE]
• 实际结果：输出的$global:limitReached无值（显示为空）

问题原因 #

Foreach-Object -Parallel会启动独立的PowerShell后台进程执行并行任务，进程间的全局变量是隔离的——你在并行块内设置的$global:limitReached只属于当前子进程，主进程无法读取到这个值。另外原脚本中$a变量在并行块内未通过using:引用，实际是未定义状态，这也是一个隐性问题。

解决方案 #

方法1：通过输出值收集结果 #

并行块内直接返回标记值，主进程收集所有并行任务的输出后判断状态：

$retries = 2
$secondsDelay = 2
$a = "house"

# 收集并行任务的输出结果
$results = 1..2 | ForEach-Object -Parallel {
    $retryCount = 0
    $ready = $false

    while ($ready -ne $true)
    {      
        if ($using:a -eq "hello")  # 用using:引用外部变量
        {
            Write-Output "a is equal `"hello`""
            $ready = $true                    
        }
        else
        {
            Write-Output "a is not equal `"hello`""

            if ($retryCount -ge $using:retries)
            {
                Write-Output "a is STILL not equal `"hello`""
                # 返回标记值
                return $true
            }
            else
            {
                Write-Output "let's give more runs.`n"
                Start-Sleep $using:secondsDelay
                $retryCount++
            }                    
        }
    }        
}

# 判断是否有任务触发重试上限
$limitReached = $results -contains $true
Write-Host "I need this value - limitReached here is [$limitReached]"

方法2：使用线程安全集合共享状态 #

如果需要共享更复杂的状态，可使用ConcurrentBag这类线程安全集合，避免多进程竞态问题：

$retries = 2
$secondsDelay = 2
$a = "house"

# 创建线程安全集合
$statusBag = [System.Collections.Concurrent.ConcurrentBag[bool]]::new()

1..2 | ForEach-Object -Parallel {
    $retryCount = 0
    $ready = $false
    $localBag = $using:statusBag

    while ($ready -ne $true)
    {      
        if ($using:a -eq "hello")
        {
            Write-Output "a is equal `"hello`""
            $ready = $true                    
        }
        else
        {
            Write-Output "a is not equal `"hello`""

            if ($retryCount -ge $using:retries)
            {
                Write-Output "a is STILL not equal `"hello`""
                # 向线程安全集合添加标记
                $localBag.Add($true)
                return
            }
            else
            {
                Write-Output "let's give more runs.`n"
                Start-Sleep $using:secondsDelay
                $retryCount++
            }                    
        }
    }        
}

# 检查集合中是否存在触发上限的标记
$limitReached = $statusBag -contains $true
Write-Host "I need this value - limitReached here is [$limitReached]"

内容的提问来源于stack exchange，提问作者Mund