Property passed to Invoke-Command changes type from IDictionary to HashTable

Question

I've been getting an error running Invoke-Command where the script block takes a parameter of type dictionary:

Cannot process argument transformation on parameter 'dictionary'. Cannot convert the "System.Collections.Hashtable" value of type "System.Collections.Hashtable" to type "System.Collections.Generic.IDictionary`2[System.String,System.String]". At line:7 char:1 + Invoke-Command -ComputerName . -ArgumentList $dictionary -ScriptBlock ... + ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + CategoryInfo : InvalidData: (:) [], ParameterBindin...mationException + FullyQualifiedErrorId : ParameterArgumentTransformationError + PSComputerName : localhost

After a lot of digging I was able to reduce the script to the the MVP below to show the root of this issue:

[System.Collections.Generic.IDictionary[string, string]]$dictionary = New-Object -TypeName 'System.Collections.Generic.Dictionary[string, string]' 
$dictionary.Add('one', 'hello')
$dictionary.Add('two', 'world')
Write-Verbose "Main Script $($dictionary.GetType().FullName)" -Verbose #outputs: VERBOSE: Before System.Collections.Generic.Dictionary`2[[System.String, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089],[System.String, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]]
Invoke-Command -ComputerName . -ArgumentList $dictionary -ScriptBlock {
    Param (
        #[System.Collections.Generic.IDictionary[string, string]] #if I leave this in I get a conversion error
        $dictionary
    )
    Write-Verbose "Function before $($dictionary.GetType().FullName)" -Verbose #outputs: VERBOSE: After System.Collections.Hashtable
    function Poc {} #this line seems to cause the `$dictionary` to become a HashTable
    Write-Verbose "Function after $($dictionary.GetType().FullName)" -Verbose #outputs: VERBOSE: After System.Collections.Hashtable

}

It seems that if the script block for Invoke-Command includes any inline functions then the parameter is automatically converted to a HashTable; whilst if the script block doesn't contain any nested function definitions the parameter is left as a System.Collections.Generic.IDictionary[string, string].

Am I misusing this feature / is there a common workaround? Or is this a just a bug in PowerShell?

Answer 1

This is a known problem with the deserialization code that is involved in PowerShell remoting (which is what Invoke-Command -ComputerName is based on), unfortunately, as of PowerShell Core 7.0.0-preview.2:

On deserialization, any object that implements the IDictionary interface is incorrectly assumed to always be a regular, non-generic [hashtable]^[1] and deserialized as such.

This is especially problematic when the original object was an ordered dictionary^[2], because the ordering of the keys is lost, as is the ability to index into them.

See GitHub issue #2861, which is tagged as "up for grabs", meaning that the community is free to offer a PR that fixes the problem.

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^{[1] System.Collections.Hashtable, creatable in PowerShell with @{ ... } literals, where both the keys and values are [object]-typed.}

^{[2] E.g., System.Collections.Specialized.OrderedDictionary, creatable in PowerShell with [ordered] @{ ... } literals, also with [object]-typed keys and values (non-generic); suprisingly, as of this writing, there is no generic ordered dictionary type - see this question.}