Why does ScriptBlock convert Hashtable to Collection and how to avoid it?

Have a look at InvokeReturnAsIs method:

[Hashtable] $t = @{} function foo($x) { $x }$t2 = foo $t$t3 = {param([Hashtable] $foo) [Hashtable]$foo}.InvokeReturnAsIs($t)Write-Host $t.GetType()Write-Host $t2.GetType()Write-Host $t3.GetType()

Which outputs:

System.Collections.HashtableSystem.Collections.HashtableSystem.Collections.Hashtable

It seems to give the result you are looking for, but the documentation does not give much informations

I believe the Hashtable gets converted to Collection due to a call of the ScriptBlock.Invoke method. Its return type is System.Collections.ObjectModel.Collection<System.Management.Automation.PSObject>.

Looks like this can be circumvented by invoking the script block via Invoke-Command:

[Hashtable] $t = @{} $t5 = Invoke-Command -ScriptBlock {param([Hashtable] $x) [Hashtable]$x} -ArgumentList $t$t6 = Invoke-Command -ScriptBlock $function:foo -ArgumentList $tWrite-Host "Invoke-Command on script block      " $t5.GetType()Write-Host "Invoke-Command on function variable " $t6.GetType()

This outputs:

Invoke-Command on script block        System.Collections.HashtableInvoke-Command on function variable   System.Collections.Hashtable