powershell - `Invoke-RestMethod -Uri “…”-方法获取|选择X，Y ` doesn' t时返回任何行，而`(Invoke-RestMethod -Uri “…”-方法Get)|选择X，Y`吗？

Question

我有一个REST API，它将返回行。然而，

为什么Invoke-RestMethod -Uri "..." -Method Get | select XXX,YYY仅返回 header ？

XXX YYY
--- ---

但是 (Invoke-RestMethod -Uri "..." -Method Get) | select X,Y返回行？

首先分配一个变量也可以。

$x = Invoke-RestMethod -Uri "..." -Method Get | select XXX,YYY
$x | select xxx,yyyy

Answer 1

一般来说:

如果命令将集合作为单个对象输出，则Select-Object X, Y将无法按预期的方式工作，因为它将在集合对象上查找无法找到它们的那些属性，在这种情况下，Select-Object将使用所请求的属性，然后所有这些属性都包含$null。

Invoke-RestMethod可能是此行为的候选者，因为它可能会通过 ConvertFrom-Json 隐式地将返回值解析为JSON，实际上会将数组作为单个对象输出；在this GitHub issue中讨论了这种令人惊讶的行为。

在命令周围放置(...)会强制枚举，这样可以解决问题:

# Place (...) around the Invoke-RestMethod call to force enumeration.
(Invoke-RestMethod -Uri "..." -Method Get) | select XXX,YY

另一个选项是分配给(中间)变量，如您的问题所示-尽管 (...)方法更简单，但是如果您实际上不需要存储中间结果。

# Store array in a variable.
$array = Invoke-RestMethod -Uri "..." -Method Get

# An array stored in a variable sent through the pipeline is 
# invariably enumerated.
$array | select X,Y

之所以可行，是因为通过管道发送存储在变量中的数组总是会枚举它(将元素逐个发送)。

通过分配给变量，您可以有效地消除一个命令一个输出一个N对象和一个将N元素数组作为单个对象输出的命令之间的区别:

# Send an array *as a whole* through the pipeline.
PS> (Write-Output -NoEnumerate (1..3) | Measure-Object).Count
1 # That is, the 3-element array was sent as *one* object

# Wrapping the command in (...) forces enumeration.
PS> ((Write-Output -NoEnumerate (1..3)) | Measure-Object).Count
3  # elements were sent *one by one*

# Store output-as-a-whole array in a variable,
# then send the variable through the pipeline - 
# which also forces enumeration.
PS> $array = Write-Output -NoEnumerate (1..3); ($array | Measure-Object).Count 
3  # elements were sent *one by one*

相反，如果确实希望通过管道发送存储在变量中的数组作为整体， ，则有两个选择:

$array = 1..3

# Use Write-Output -NoEnumerate:
PS> (Write-Output -NoEnumerate $array | Measure-Object).Count
1 # array was sent *as a whole*

# Alternatively - faster and more concise, but more obscure -
# wrap the array in an aux. wrapper array, so that only 
# the wrapper array is enumerated, sending the original array
# as a whole:
PS> (, $array | Measure-Object).Count
1 # array was sent *as a whole*