PowerShell中ValidateScript的生命周期及验证失效原因探究

PowerShell支持为变量配置验证脚本（ValidateScript），该功能最常用于验证函数参数，也可应用于任意变量。这类验证脚本会在变量值每次变更时自动执行。

# 创建带验证脚本的变量
[ValidateScript({
    if (-not ($_.Length -in @(8, 16, 24))) {
        Write-Host 'Validate Failed'
        throw "Value '$_' has invalid length: $($_.Length)"
    } else {
        Write-Host 'Validate Success'
    }
    $true
})][System.String]$MyValidatedArg = "a" * 8 # 输出 "Validate Success"

# 以下赋值操作都会触发验证脚本
Write-Host "Several valid assignments..."
$MyValidatedArg = "b" * 8 # 输出 "Validate Success"
$MyValidatedArg = "b" * 8 # 输出 "Validate Success"
$MyValidatedArg = "b" * 16 # 输出 "Validate Success"
$MyValidatedArg = "b" * 24 # 输出 "Validate Success"

若为变量赋无效值，会直接抛出异常：

$MyValidatedArg = "foo" # 抛出异常

奇怪的是，在赋值时重新指定类型约束可以绕过验证脚本，甚至会导致后续的常规赋值也不再触发验证。已知“避免这种操作”是一种解决方式，但核心疑问是：为何这种操作可行？

[System.String]$MyValidatedArg = "123456789" # 值无效但执行成功

最初猜测这是变量遮蔽问题，即重新指定类型约束时可能创建了第二个变量。为此尝试捕获变量的引用，查看引用是否保留验证脚本，但结果并非如此（对PowerShell内部机制不熟悉，不确定该测试是否有效）：

# 创建变量引用并通过引用赋值
Write-Host "Assigning through reference..."
$refToParameter = [ref] $MyValidatedArg
$refToParameter.Value = "c" * 24 # 输出 "Validate Success"
Write-Host " Read via variable: $MyValidatedArg"
Write-Host " Read via reference: $($refToParameter.Value)"

# 引用与变量关联，任意一方赋值都会触发验证脚本
Write-Host "Assigning through normal variable again..."
$MyValidatedArg = "d" * 24 # 输出 "Validate Success"
Write-Host " Read via variable: $MyValidatedArg"
Write-Host " Read via reference: $($refToParameter.Value)"

# 以下操作成功执行，不符合预期
Write-Host "Assigning with [System.String]..."
[System.String]$MyValidatedArg = "123456789" # 值无效但无输出、无异常
Write-Host " No exceptions yet!"

# 并非变量遮蔽，引用也同步了新值
Write-Host "The link between the variable and the reference is still there..."
$MyValidatedArg = "e" * 8 # 值有效但无输出
Write-Host " Read via variable: $MyValidatedArg"
Write-Host " Read via reference: $($refToParameter.Value)"

# 此时验证脚本已失效
Write-Host "Regular invalid assignment..."
$MyValidatedArg = "123456789" # 值无效但无输出、无异常
Write-Host " It works now for some reason"

综上，需要明确：

• PowerShell中ValidateScript的生命周期是怎样的？
• 为何重新指定类型约束的赋值操作会绕过验证，并导致后续验证脚本彻底失效？

内容的提问来源于stack exchange，提问作者9072997