使用Pester测试Get-EvenNumbers函数出现返回类型错误的问题

问题背景 #

我正在学习Pester，使用ChatGPT生成的Get-EvenNumbers函数及对应的Pester测试代码，执行测试时出现类型不匹配错误，但我认为函数返回的应该是[int]数组，想知道错误原因。

Get-EvenNumbers函数代码 #

function Get-EvenNumbers {
param(
[Parameter(Mandatory=$true)]
[int]$start,

[Parameter(Mandatory=$true)]
[int]$end
)

$evenNumbers = @()

for ($i = $start; $i -le $end; $i++) {
    if ($i % 2 -eq 0) {
      $evenNumbers += $i
    }
}

return $evenNumbers
}

Pester测试代码 #

Describe "Get-EvenNumbers" {
Context "When the input range is from 1 to 10" {
$start = 1
$end = 10

It "Returns an array of even numbers" {
  $result = Get-EvenNumbers -Start $start -End $end
  $result | Should -BeOfType "System.Array"
  $result | Should -Not -BeNullOrEmpty
  $result | Should -ContainOnly (2, 4, 6, 8, 10)
   }
}

Context "When the input range is from 2 to 9" {
$start = 2
$end = 9

It "Returns an array of even numbers" {
  $result = Get-EvenNumbers -Start $start -End $end
  $result | Should -BeOfType "System.Array"
  $result | Should -Not -BeNullOrEmpty
  $result | Should -ContainOnly (2, 4, 6, 8)
     }
    }
}

测试错误信息 #

[-] Get-EvenNumbers.When the input range is from 1 to 10.Returns an array of even numbers 10ms (8ms|1ms)
Expected the value to have type [array] or any of its subtypes, but got 2 with type [int].
at $result | Should -BeOfType "System.Array", C:\Users\Pester\Get-EvenNumbers.Tests.ps1:8
at , C:\Users\Pester\Get-EvenNumbers.Tests.ps1:8
[-] Get-EvenNumbers.When the input range is from 2 to 9.Returns an array of even numbers 13ms (9ms|4ms)
Expected the value to have type [array] or any of its subtypes, but got 2 with type [int].
at $result | Should -BeOfType "System.Array", C:\Users\Pester\Get-EvenNumbers.Tests.ps1:20
at , C:\Users\Pester\Get-EvenNumbers.Tests.ps1:20
Tests completed in 177ms
Tests Passed: 0, Failed: 2, Skipped: 0 NotRun: 0

错误原因分析 #

这是PowerShell的管道遍历特性导致的：

• 当你把数组通过管道传给Should -BeOfType时，PowerShell会自动遍历数组中的每个元素，对单个元素进行类型检查，而不是检查整个变量的数组类型。错误信息里提到的“got 2 with type [int]”，就是因为它拿数组的第一个元素（2）去和System.Array类型做对比，自然不匹配。

修正方案 #

有两种简单的解决方式：

1. 直接传递变量参数：不使用管道，而是通过参数传递整个变量，让Pester检查变量的整体类型：

   # 原代码
   $result | Should -BeOfType "System.Array"
   # 修改后
   Should -BeOfType -ActualValue $result -ExpectedType "System.Array"
   

2. 使用更准确的类型断言：可以直接检查是否为数组类型，或者匹配具体的[int[]]类型：

   # 检查是否为数组
   $result | Should -IsArray
   # 或者检查具体的整数数组类型
   $result | Should -BeOfType [int[]]
   

调整后就能正确验证函数返回值是数组类型，测试也能通过。

内容的提问来源于stack exchange，提问作者Purclot