Powershell 未显示 PSCustomObjects（或其他对象）的某些属性

Question

Originally, this question was prompted while investigating an answer to question 27373126 about selecting items from an array of hash tables. (This was about shorter ways to get the properties than using Foreach-Object or Select-Object with hash table arguments.)

Having found that I could get items using script-block arguments to Select-Object as follows:

$hash = @{title="title1";detail="detail1"},@{title="title2";detail="detail2"}
$hash | select-object {$_.title},{$_.detail}

and that I could tidy up the output objects (produced with properties named "$_.title" and "$_.detail") by defining functions as follows:

function title{$_.title};function detail{$_.detail}
$hash | select {title},{detail}

I then tried to automate producing output with properties defined by a list of property names (I know these are wrong, this is just how I found the issue I am questioning):

'title','detail' | foreach { $hash | select $_ }

title
-----





# 4 blank lines

I know why there are 4 blank lines. There is no "title" member in $hash , it is a "title" item . The surprising part was no column for "detail". Tried this:

'title','detail' | foreach { $hash | select $_ } | gm

   TypeName: Selected.System.Collections.Hashtable

Name        MemberType   Definition
----        ----------   ----------
Equals      Method       bool Equals(System.Object obj)
GetHashCode Method       int GetHashCode()
GetType     Method       type GetType()
ToString    Method       string ToString()
title       NoteProperty object title=null

Maybe something strange with Foreach-Object providing arguments to Select-Object . Tried:

$hash | select title; $hash|select detail

title
-----





# 4 blank lines

Which command is generating the output?:

'start';$hash|select title;'in';$hash|select detail;'done'
start

title
-----


in


done

So 2 blank lines from each section but no "detail" member. Time to get a bit more specific:

'start';$hash|select title|gm;'in';$hash|select detail|gm;'done'
start


   TypeName: Selected.System.Management.Automation.Internal.Host.InternalHost

Name        MemberType   Definition
----        ----------   ----------
Equals      Method       bool Equals(System.Object obj)
GetHashCode Method       int GetHashCode()
GetType     Method       type GetType()
ToString    Method       string ToString()
title       NoteProperty object title=null
in
Equals      Method       bool Equals(System.Object obj)
GetHashCode Method       int GetHashCode()
GetType     Method       type GetType()
ToString    Method       string ToString()
detail      NoteProperty object detail=null
done

So the second select IS generating a "detail" member but it is getting lost somewhere. How about this:

&{$hash|select title;$hash|select detail}|gm


   TypeName: Selected.System.Management.Automation.Internal.Host.InternalHost

Name        MemberType   Definition
----        ----------   ----------
Equals      Method       bool Equals(System.Object obj)
GetHashCode Method       int GetHashCode()
GetType     Method       type GetType()
ToString    Method       string ToString()
title       NoteProperty object title=null

No "detail" again. After lots of fruitless testing, some useful research uncovered a partial answer. The full description is in How PowerShell Formatting and Outputting REALLY works but the upshot is that the final display was being controlled by the properties of the first object through the pipeline. This object had a "title" property but no "detail" property so the output was formatted to match. That explains why the displayed output had no "detail" column but why did the last Get-Member not show it? Get-Member should have received 2 objects with a "title" followed by 2 objects with a "detail". I know this because a manual Get-Member does show them:

&{$hash|select title;$hash|select detail}|foreach{[componentmodel.typedescriptor]::getproperties($_)[0].name}
title
title
detail
detail

# Can't use gettype() since doesn't know about added properties

So it would seem that not just Format-Table decides its output based on the first object down the pipeline. Apparently, Get-Member also exhibits this behaviour, though in this case it is the first object of each type . All the objects were PSCustomObjects but the first had only a "title" added property so Get-Member assumed that all PSCustomObjects looked the same.

And finally, THE QUESTION . Which other cmdlets, if any, exhibit this "first object down the pipeline silently defines subsequent behaviour" functionality?

Answer 1

^{Note: In PowerShell versions before 7.0 , hash tables indeed aren't supported as input to Select-Object ; in earlier versions, cast them to [pscustomobject] first; eg:
[pscustomobject] @{title="title1";detail="detail1"}, [pscustomobject] @{title="title2";detail="detail2"} | Select-Object title, detail}

To recap the problem that you're already aware of: (possibly implicit) Format-Table output locks in the properties of the first object as display columns and subsequent objects that do not have these properties will show blank column values - see this answer for details.
It is important to note that this is merely a display problem.

The Get-Member problem is slightly different: it isn't about the first input object's type, it's about input objects that share the same type : Get-Member is designed to produce output for each distinct type among the input objects.

The problem is that two [pscustomobject] instances (such as output by Select-Object ) are considered the same type by Get-Member (which, from a .NET perspective, they technically are ) - even if they have different properties , so Get-Member will in effect only show the members of the first [pscustomobject] instance among the input objects (which may or may not be the first input object overall ).

Therefore, in order to pipe multiple [pscustomobject] instances to Get-Member and see the properties of each , call Get-Member via ForEach-Object :

# Two sample [pscustomobject] instances with non-overlapping properties.
$objs = [pscustomobject] @{ foo = 1 }, [pscustomobject] @{ bar = 2 }
# Call Get-Member on *each* object.
$objs | ForEach-Object { Get-Member -InputObject $_ }

---

As for:

THE QUESTION. Which other cmdlets, if any, exhibit this "first object down the pipeline silently defines subsequent behaviour" functionality?

In addition to (potentially implicitly applied) Format-Table , in essence all cmdlets that expect all input objects to be of the same type (have the same set of properties) exhibit the behavior , notably Export-Csv / ConvertTo-Csv . Any number of third-party cmdlets may fall into that category as well - there's no way to come up with an exhaustive list, not least because new cmdlets may be introduced in the future.

You can generally infer from the purpose of a cmdlet whether it requires uniform input or not (and, hopefully, the documentation makes it clear too):

• Export-Csv and ConvertTo-Csv , because they create tabular data, require uniform input; while you can use -Force with -Append to make Export-Csv append objects that also have different properties to a preexisting CSV file, only those properties present on the newly added objects that match the preexisting columns (properties) are added, so uniformity is ultimately still enforced.

• By contrast, ConvertTo-Json , because it can serialize arbitrary object graphs , does not require uniform input.