为什么此字符串不在 PowerShell 中序列化为简单的 JSON 字符串 [英] Why doesn't this string serialize to a simple JSON string in PowerShell
问题描述
见下文,$a
和 $s
都是包含文本 String"
的字符串,但每个字符串都使用 ConvertTo-JSON 进行不同的序列化.
See below both $a
and $s
are strings containing the text "String"
but each serializes differently with ConvertTo-JSON.
为什么不 $s |ConvertToJson
产生 "String"
??
Why won't $s | ConvertToJson
produce "String"
??
PS W:\PowerShell\powowshell> $a="String"
PS W:\PowerShell\powowshell> $a
String
PS W:\PowerShell\powowshell> $a.gettype()
IsPublic IsSerial Name BaseType
-------- -------- ---- --------
True True String System.Object
PS W:\PowerShell\powowshell> $a | ConvertTo-Json
"String"
PS W:\PowerShell\powowshell> $s
String
PS W:\PowerShell\powowshell> $s.gettype()
IsPublic IsSerial Name BaseType
-------- -------- ---- --------
True True String System.Object
PS W:\PowerShell\powowshell> $s | ConvertTo-Json
{
"value": "String",
"required": "true"
}
背景故事
$s
是使用 Get-Help
检查的 .ps1
的 parameterValue
:
Back-Story
$s
is the parameterValue
of a .ps1
inspected with Get-Help
:
PS W:\PowerShell\powowshell> $cmd = (get-help -full W:\PowerShell\powowshell\examples\components\dosdir.ps1).Syntax.syntaxItem[0].parameter
PS W:\PowerShell\powowshell> $cmd | convertto-json
{
"description": [
{
"Text": "The path to the directory to be listed"
}
],
"parameterValue": {
"value": "String",
"required": "true"
},
...
$s = $cmd.parameterValue
dosdir.ps1:
param(
[String]$Path
)
CMD /C "DIR /B $Path"
推荐答案
PowerShell 的 ETS(扩展类型系统) 允许您使用附加属性(只能由 PowerShell 代码直接访问)来修饰任何对象.
PowerShell's ETS (Extended Type System) allows you to decorate any object with additional properties (that are directly accessible only to PowerShell code).
如果你使用 [string]
实例(无论是你自己做还是其他命令为你做的[1]),这些额外的属性就会出现当对象使用 ConvertTo-Json
序列化时:
If you do so with a [string]
instance (whether you do it yourself or another command does it for you[1]), these additional properties will surface when the object is serialized with ConvertTo-Json
:
# Add a .foo property with value 'bar' to a string.
$decoratedString = 'hi' | Add-Member -PassThru foo bar
# Output the string as-is.
# The added property does NOT show.
$decoratedString
'---'
# Serialize the string to JSON.
# The added property DOES show and the string's actual content
# is presented as pseudo-property .value
$decoratedString | ConvertTo-Json
以上产生:
hi
---
{
"value": "hi",
"foo": "bar"
}
这个 GitHub 问题讨论了这种令人惊讶的行为.
This GitHub issue discusses this surprising behavior.
解决方法:
# .psobject.BaseObject returns the underlying, undecorated object.
PS> $decoratedString.psobject.BaseObject | ConvertTo-Json
hi
[1] 正如 js2010 指出的那样,数据检索 PowerShellprovider cmdlets - Get-ChildItem
, Get-Item
, Get-Content
, ... - 都添加了固定数量的NoteProperty
成员为它们输出的对象,即PSPath
、PSParentPath
、PSChildName
、PSDrive
>, PSProvider
.
[1] As js2010 points out, the data-retrieving PowerShell provider cmdlets - Get-ChildItem
, Get-Item
, Get-Content
, ... - all add a fixed number of NoteProperty
members to the objects they output, namely PSPath
, PSParentPath
, PSChildName
, PSDrive
, PSProvider
.
因此,如果您对使用 Get-Content
获得的字符串进行序列化,则会遇到上面详述的相同问题:
Therefore, you'll run into the same problem detailed above if you serialize a string that was obtained with Get-Content
:
PS> 'hi' > t.txt; Get-Content t.txt | ConvertTo-Json
{
"value": "hi",
"PSPath": "/Users/jdoe/t.txt",
"PSParentPath": "/Users/jdoe",
"PSChildName": "t.txt",
"PSDrive": {
"CurrentLocation": "Users/jdoe",
"Name": "/",
"Provider": {
"ImplementingType": "Microsoft.PowerShell.Commands.FileSystemProvider",
"HelpFile": "System.Management.Automation.dll-Help.xml",
"Name": "FileSystem",
"PSSnapIn": "Microsoft.PowerShell.Core",
...
请注意,在字符串的情况下,这些额外的属性在构造 new 字符串时丢失,通过字符串连接或应用字符串运算符(例如 <代码>-替换:
Note that in the case of a string these extra properties are lost when a new string is constructed, either by string concatenation or by applying a string operator such as -replace
:
# String concatenation
PS> 'hi' > t.txt; (Get-Content t.txt) + '!' | ConvertTo-Json
hi!
# Using -replace
PS> (Get-Content t.txt) -replace 'i', 'o' | ConvertTo-Json
ho
另请注意,这种每个输出对象的装饰增加了相当多的内存和性能开销;对于 Get-Content
,GitHub issue #7537 建议提供选择退出.
Also note that this per-output-object decorating adds quite a bit of memory and performance overhead; for Get-Content
, GitHub issue #7537 suggests offering an opt-out.
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