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

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

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 

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...