Null List in Pester Mock created from a closure

Question

Why does the following code:

  function CreateExecutedCommandsLogger(){
     param(
        [System.Collections.Generic.List[System.String]]$cmdLog
     )
     "WWWWW=>{0}" -f $cmdLog | Write-Host
     return{
        param(
           [parameter(valuefrompipeline)]$command
        )
        "XXXXX=>{0}" -f $command | Write-Host
        "YYYYY=>{0}" -f $cmdLog | Write-Host
        $cmdLog.Add($command)
     }.GetNewClosure()
  }

  $executedCommands = New-Object System.Collections.Generic.List[System.String]
  Mock ExecuteSqlCommand (CreateExecutedCommandsLogger $executedCommands)

Result in the error:

  RuntimeException: You cannot call a method on a null-valued expression.
  at <ScriptBlock>, <No file>: line 7
  at <ScriptBlock>, C:\Users\notme\Documents\WindowsPowerShell\Modules\Pester\Functions\Mock.ps1: line 1018
  at ExecuteBlock, C:\Users\notme\Documents\WindowsPowerShell\Modules\Pester\Functions\Mock.ps1: line 1022
  at Invoke-Mock, C:\Users\notme\Documents\WindowsPowerShell\Modules\Pester\Functions\Mock.ps1: line 868
  at <ScriptBlock><Process>, <No file>: line 53

For reference the tracing output gives:

WWWWW=>System.Collections.Generic.List`1[System.String]
XXXXX=>C:\dummy\command.exe -S "blah" -d "blahblah" -i "something.sql"
YYYYY=>

When I use the following, it works:

  $dummyMock = (CreateExecutedCommandsLogger $executedCommands)
  &$dummyMock "blah"

I'm assuming this is something to do with the way the script block in the mock is executed?

Answer 1

As you can see from source (Get-Command Mock).ScriptBlock , there is single line where $MockWith parameter is used:

$mockWithCopy = [scriptblock]::Create($MockWith.ToString())

So that, $MockWith effectively is a string and any closure ignored. Likely it is done because on next line newly created script block $mockWithCopy bounded to some session state, thus you loose your closure anyway:

Set-ScriptBlockScope -ScriptBlock $mockWithCopy -SessionState $contextInfo.Session

Pester first do private copy of script block to not affect passed instance.