Method .GetType() returns different values for the same variable - powershell

Question

Practicing with the . GetType() method, I have found that if I write a function that returns 'Name' and 'BaseType' I get a different result than expected when I do not write the function.
Example:
function Get-BaseType_Name {
param(
[Parameter(Mandatory,ValuefromPipeline=$True)]
[System.Object]$var
)
$var.GetType() | Select -Property Name, BaseType;
}
$var = Get-Service
$var | Get-BaseType_Name
I get:
Name BaseType
---- --------
ServiceController System.ComponentModel.Component
However, if I execute the sentences:
$var = Get-Service
$var.GetType() | Select -Property Name, BaseType;
I get:
Name BaseType
---- --------
Object[] System.Array
Why is that?
I expected the same result in both cases

Answer 1

Per #MatthiasRJessen's answer, you're passing service objects into your function one-at-a-time via the pipeline.
There's a little bit more to it though - the reason you're only seeing one output result is because your function is equivalent this:
function Get-BaseType_Name {
param(
[Parameter(Mandatory,ValuefromPipeline=$True)]
[System.Object]$var
)
END {
#^^^^ END BLOCK
$var.GetType() | Select -Property Name, BaseType;
}
}
That is, your function body is being treated as an end block, which you can confirm if you look at the AST that PowerShell generates for your code:
${function:Get-BaseType_Name}.Ast.Body
which produces the following output.
(Note that the BeginBlock and ProcessBlock are empty and your code appears in the EndBlock).
Attributes : {}
UsingStatements : {}
ParamBlock : param(
[Parameter(Mandatory,ValuefromPipeline=$True)]
[System.Object]$var
)
BeginBlock :
^^^^^^^^^^ no definition
ProcessBlock :
^^^^^^^^^^^^ no definition
EndBlock : param(
[Parameter(Mandatory,ValuefromPipeline=$True)]
[System.Object]$var
)
$var.GetType() | Select -Property Name, BaseType
^^^^^^^^ your code is in this block
DynamicParamBlock :
ScriptRequirements :
Extent : {
param(
[Parameter(Mandatory,ValuefromPipeline=$True)]
[System.Object]$var
)
$var.GetType() | Select -Property Name, BaseType;
}
Parent : function Get-BaseType_Name {
param(
[Parameter(Mandatory,ValuefromPipeline=$True)]
[System.Object]$var
)
$var.GetType() | Select -Property Name, BaseType;
}
If you have a look at the end section in about_Functions_Advanced_Methods it says:
end
This block is used to provide optional one-time post-processing for the function.
What's happening is PowerShell is feeding each of the results from Get-Service into the Get-BaseType_Name function one-at-a-time, but your code is only actually doing anything once the last item has been processed, and in that case $var holds a reference to the last item, which you can see if you run this instead:
function Get-BaseType_Name {
param(
[Parameter(Mandatory,ValuefromPipeline=$True)]
[System.Object]$var
)
write-host $var.Name
#^^^^^^^^^^^^^^^^^^^ -- write the name of the service to the console
$var.GetType() | Select -Property Name, BaseType;
}
Get-Service | Get-BaseType_Name
On my machine I see this output:
XboxNetApiSvc
Name BaseType
---- --------
ServiceController System.ComponentModel.Component
where XboxNetApiSvc is the name of the last service returned by Get-Service.
If you want to see the output for every item you can put your code in the process block instead:
function Get-BaseType_Name {
param(
[Parameter(Mandatory,ValuefromPipeline=$True)]
[System.Object]$var
)
PROCESS {
#^^^^^^^^ PROCESS BLOCK
$var.GetType() | Select -Property Name, BaseType;
}
}
Get-Service | Get-BaseType_Name
and then your optput will look like this:
Name BaseType
---- --------
ServiceController System.ComponentModel.Component
ServiceController System.ComponentModel.Component
ServiceController System.ComponentModel.Component
... etc ...
In summary, you're sending a collection of service through the pipeline (per #MathiasRJessen's answer), but you're only returning values for the last item in that collection because PowerShell is implicitly treating your function body as an end block...

Answer 2

From the about_Pipelines help topic:
One-at-a-time processing
[...]
When you pipe multiple objects to a command, PowerShell sends the objects to the command one at a time. When you use a command parameter, the objects are sent as a single array object. This minor difference has significant consequences.
When executing a pipeline, PowerShell automatically enumerates any type that implements the IEnumerable interface and sends the members through the pipeline one at a time. The exception is [hashtable], which requires a call to the GetEnumerator() method.
In other words: this is by design - PowerShell sees that $var contains an array (a type that implements the IEnumerable interface), and starts enumerating the items in it one-by-one
You can prevent automatic enumeration by using Write-Output -NoEnumerate:
PS ~> Write-Output $var -NoEnumerate | Get-BaseType_Name
Name BaseType
---- --------
Object[] System.Array