$MyInvocation.MyCommand.Name becomes null in Where-Object block

Issue description: Unable to use $MyInvocation.MyCommand.Name in Where-Object block as it becomes null. ForEach and For-EachObject works fine. I see this only with Where-Object.

I understand there are workarounds but just want to know if this is expected behavior or not as I was not able to find document that describes scope in Where-Object.

Steps to reproduce

Copy the following script to test.ps1

1 | Where-Object{
    Write-Host('MyInvocation in Where-Object: ' + $MyInvocation.MyCommand.Name)
}

1 | ForEach-Object{
    Write-Host('MyInvocation in ForEach-Object: ' + $MyInvocation.MyCommand.Name)
}

ForEach($FileName in 1){
    Write-Host('MyInvocation in ForEach: ' + $MyInvocation.MyCommand.Name)
}

Then run

.\test.ps1

Expected behavior

## Expected output in case script name is test.ps1.

MyInvocation in Where-Object: test.ps1
MyInvocation in ForEach-Object: test.ps1
MyInvocation in ForEach: test.ps1

Actual behavior

## Expected output in case script name is test.ps1.

MyInvocation in Where-Object:       <-------- $MyInvocation.MyCommand.Name becomes null 
MyInvocation in ForEach-Object: test.ps1
MyInvocation in ForEach: test.ps1

Environment data

> $PSVersionTable

Name Value
---- -----
PSVersion 6.2.1
PSEdition Core
GitCommitId 6.2.1
OS Microsoft Windows 10.0.18362
Platform Win32NT
PSCompatibleVersions {1.0, 2.0, 3.0, 4.0…}
PSRemotingProtocolVersion 2.3
SerializationVersion 1.1.0.1
WSManStackVersion 3.0

[daxian-dbw]

I agree. We need to decide which behavior is the correct one. From my personal view, Where-Object seems doing the right thing because just like @BrucePay said, a script block is an anonymous function.

Here is another example:

PS> cat .\b.ps1
. { Write-Host('MyInvocation in the script block: ' + $MyInvocation.MyCommand.Name) }
PS> .\b.ps1
MyInvocation in the script block:

Here, b.ps1 contains an invocation of a script block, and as we can see, the script block doesn’t uses b.ps1 as the command name.

#10454 is related to this issue. The PR made an optimization to override ForEahc-Object with dot-sourcing a filter-like script block for some common uses of ForEach-Object, and when that optimization kicks in, $MyInvocation.MyCommand.Name is null for ForEach-Object too. Whether that should be fixed and how to fix it depends on the conclusion of this issue.

---

Here are the output of $MyInvocation from Where-Object and ForEach-Object in the repro scenario as in preview.3:

MyInvocation in ForEach-Object:

MyCommand             : a.ps1
BoundParameters       : {}
UnboundArguments      : {}
ScriptLineNumber      : 1
OffsetInLine          : 1
HistoryId             : 46
ScriptName            :
Line                  : .\a.ps1
PositionMessage       : At line:1 char:1
                        + .\a.ps1
                        + ~~~~~~~
PSScriptRoot          :
PSCommandPath         :
InvocationName        : .\a.ps1
PipelineLength        : 1
PipelinePosition      : 1
ExpectingInput        : False
CommandOrigin         : Runspace
DisplayScriptPosition :

MyInvocation in Where-Object:

MyCommand             :
BoundParameters       : {}
UnboundArguments      : {}
ScriptLineNumber      : 1
OffsetInLine          : 1
HistoryId             : 47
ScriptName            :
Line                  : .\c.ps1
PositionMessage       : At line:1 char:1
                        + .\c.ps1
                        + ~~~~~~~
PSScriptRoot          :
PSCommandPath         :
InvocationName        :
PipelineLength        : 0
PipelinePosition      : 0
ExpectingInput        : False
CommandOrigin         : Internal
DisplayScriptPosition :

Here is the output of $MyInvocation from ForEach-Object in the repro scenario with the optimization change in #10454:

MyInvocation in ForEach-Object:

MyCommand             :
                            'MyInvocation in ForEach-Object: '
                            $MyInvocation | fl *

BoundParameters       : {}
UnboundArguments      : {}
ScriptLineNumber      : 5
OffsetInLine          : 36
HistoryId             : 32
ScriptName            : F:\tmp\a.ps1
Line                  : 1..2 | Where-Object { $_ -gt 1 } | ForEach-Object {

PositionMessage       : At F:\tmp\a.ps1:5 char:36
                        + 1..2 | Where-Object { $_ -gt 1 } | ForEach-Object {
                        +                                    ~~~~~~~~~~~~~~~~
PSScriptRoot          : F:\tmp
PSCommandPath         : F:\tmp\a.ps1
InvocationName        : ForEach-Object
PipelineLength        : 2
PipelinePosition      : 2
ExpectingInput        : True
CommandOrigin         : Internal
DisplayScriptPosition :

[SteveL-MSFT]

@PowerShell/powershell-committee reviewed this. Inspecting the code, it appears that both Where-Object and ForEach-Object both have code to set MyInvocation, but using different APIs. It seems that user found utility in getting this information so we recommend that Where-Object should behave like ForEach-Object even if it is technically correct that it’s an anonymous function.