BSonPoSH

I often to get asked what I mean by "Select-Object changes the object type." I believe this one of those things that is easier to illustrate than just explain. First let me show the object type Change

PS> ($dir = Get-item C:\temp).Gettype()
 
IsPublic IsSerial Name                                     BaseType
-------- -------- ----                                     --------
True     True     DirectoryInfo                            System.IO.FileSystemInfo
 
PS> ($dir = Get-item C:\temp | select-Object Fullname,Name,Parent).Gettype()
 
IsPublic IsSerial Name                                     BaseType
-------- -------- ----                                     --------
True     False    PSCustomObject                           System.Object
 
Even if you specifiy PSBASE you have new object type.
 
PS> ($dir = Get-item C:\temp).psbase.Gettype()
 
IsPublic IsSerial Name                                     BaseType
-------- -------- ----                                     --------
True     True     DirectoryInfo                            System.IO.FileSystemInfo
 
PS> ($dir = Get-item C:\temp | select-Object Fullname,Name,Parent).psbase.Gettype()
 
IsPublic IsSerial Name                                     BaseType
-------- -------- ----                                     --------
True     False    PSCustomObject                           System.Object

Now look at what you lose using this method.

First you will notice you have 21 items returned from Get-Member (I only included Properties for simplicity, but methods work the same way.)

PS> get-item C:\temp | Get-Member -MemberType Properties | ft Name
 
Name
----
PSChildName
PSDrive
PSIsContainer
PSParentPath
PSPath
PSProvider
Attributes
CreationTime
CreationTimeUtc
Exists
Extension
FullName
LastAccessTime
LastAccessTimeUtc
LastWriteTime
LastWriteTimeUtc
Name
Parent
Root
Mode
ReparsePoint
 
## After piping to Select-Object you now only have 3
 
PS> Get-Item C:\temp | Select-Object Name,Fullname,Parent | Get-Member -MemberType Properties | ft name
 
Name
----
FullName
Name
Parent
 
## Now.. methods before "Conversion"
 
PS>  Get-item C:\temp | Get-Member -MemberType Methods | ft name
 
Name
----
Create
CreateObjRef
CreateSubdirectory
Delete
Equals
GetAccessControl
GetDirectories
GetFiles
GetFileSystemInfos
GetHashCode
GetLifetimeService
GetObjectData
GetType
get_Attributes
get_CreationTime
get_CreationTimeUtc
get_Exists
get_Extension
get_FullName
get_LastAccessTime
get_LastAccessTimeUtc
get_LastWriteTime
get_LastWriteTimeUtc
get_Name
get_Parent
get_Root
InitializeLifetimeService
MoveTo
Refresh
SetAccessControl
set_Attributes
set_CreationTime
set_CreationTimeUtc
set_LastAccessTime
set_LastAccessTimeUtc
set_LastWriteTime
set_LastWriteTimeUtc
ToString
 
## After
 
PS>  Get-item C:\temp | select-object FullName,Name,Parent | Get-Member -MemberType Methods | ft name
 
Name
----
Equals
GetHashCode
GetType
ToString

As you can see... Effectively they are all gone on the new object. When I first experienced this.. I thought maybe the methods were just hidden... but try to access them

PS> $dir = Get-Item C:\temp
PS> $dir.GetDirectories()
 
Mode           LastWriteTime       Length Name
----           -------------       ------ ----
d----     5/22/2007  1:58 PM              Console
d----     5/10/2007  4:36 PM              test32
 
PS> $dir = Get-Item C:\temp | Select-Object FullName,Name,Parent
PS>  $dir.GetDirectories()
Method invocation failed because [System.Management.Automation.PSCustomObject] doesn't contain a method named 'GetDirectories'.
At line:1 char:20

I want to be clear... This is not a bug. Its not even a bad idea. I find it useful, but I think it confuses a lot of people. Like most people I didn't read the help before I used it or I would have seen this excerpt from get-help Select-Object:

If you use Select-Object to select specified properties, it copies the values of those properties from the input objects and creates new objects that have the specified properties and copied values.

So, the moral of the story is don't be shocked when your object changes after using Select-Object. Embrace the change :)

tshell :: Jun.10.2009 :: Active Directory, All :: No Comments »

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