In PowerShell 7 (Preview 5), Select-Object cannot access built-in hashtable members and throws an exception instead.
With hashtables, there are two sets of properties: the built-in properties like Keys and Count, and the dynamically added properties (the actual keys). Select-Object apparently cannot access the built-in properties.
Expected Behavior
In the example below, it correctly retrieves the content of property Version:
$host
The result:
Name : ConsoleHost
Version : 7.0.0-preview.5
InstanceId : bdc67c11-8fe0-40e8-90fa-46326c9d0095
UI : System.Management.Automation.Internal.Host.InternalHostUserInterface
CurrentCulture : en-US
CurrentUICulture : en-US
PrivateData : Microsoft.PowerShell.ConsoleHost+ConsoleColorProxy
DebuggerEnabled : True
IsRunspacePushed : False
Runspace : System.Management.Automation.Runspaces.LocalRunspace
$host | Select-Object -ExpandProperty Version
The result:
Major Minor Build Revision
----- ----- ----- --------
7 0 0 -1
Problem with Hashtable Members
When you create a hashtable, it has a built-in property called Keys.
$PSVersionTable is a built-in variable which contains a hashtable that can serve as an example:
$PSVersionTable
The result:
Name Value
---- -----
PSVersion 7.0.0-preview.5
PSEdition Core
GitCommitId 7.0.0-preview.5
OS Microsoft Windows 10.0.18363
Platform Win32NT
PSCompatibleVersions {1.0, 2.0, 3.0, 4.0…}
PSRemotingProtocolVersion 2.3
SerializationVersion 1.1.0.1
WSManStackVersion 3.0
$PSVersionTable.Keys
The result:
PSVersion
PSEdition
GitCommitId
OS
Platform
PSCompatibleVersions
PSRemotingProtocolVersion
SerializationVersion
WSManStackVersion
When you look at the content of Keys, it correctly lists the keys of the hashtable.
Built-In Members are Affected
When you try and do the same with Select-Object, it cannot find the property:
$PSVersionTable | Select-Object -ExpandProperty Keys
The result:
Select-Object: Property "Keys" cannot be found.
The same applies for other built-in hashtable members such as Count. The behavior in Windows PowerShell is:
$PSVersionTable | Select-Object -ExpandProperty Keys
The result:
PSVersion
PSEdition
PSCompatibleVersions
BuildVersion
CLRVersion
WSManStackVersion
PSRemotingProtocolVersion
SerializationVersion
The same is true for the parameter -Property:
$PSVersionTable | Select-Object -Property Keys, Count
The result:
Keys Count
---- -----
Again, the built-in properties are not accessible. The behavior on Windows PowerShell is:
$PSVersionTable | Select-Object -Property Keys, Count
The result:
Keys Count
---- -----
{PSVersion, PSEdition, PSCompatibleVersions, BuildVersion...} 8
Hashtable Keys are not Affected
Select-Object works as expected with hashtable keys:
# create a hashtable:
$hash = @{}
# built-in members are not found:
$hash | Select-Object -ExpandProperty Keys
The result:
Select-Object: Property "Keys" cannot be found.
# add keys:
$hash.Name = "Tobias"
# hashtable keys are found:
$hash | Select-Object -ExpandProperty Keys
$hash | Select-Object -ExpandProperty Name
The result:
Tobias
Conclusion
Apparently, there is a bug in the code that accesses hashtable properties: the code does not appropriately combine the properties from the hashtable keys with the built-in properties.
The code solely looks at the keys and misses (shadows in some sense) the built-in members.
Workaround
Until this is fixed, a workaround for -ExpandProperty is to use Foreach-Object instead:
$PSVersionTable | ForEach-Object { $_.Keys }
The result:
PSVersion
PSEdition
GitCommitId
OS
Platform
PSCompatibleVersions
PSRemotingProtocolVersion
SerializationVersion
WSManStackVersion
There is no workaround for the parameter -Property.
Issue
This was opened as issue. See there for details on progress.