Convert default parameter value of string to type array

Question

Suppose you have the following function:

Function Test-Function {
    Param (
        [String[]]$ComputerNames = @($env:COMPUTERNAME, 'PC2'),
        [String]$PaperSize = 'A4'
    )
}

Get-DefaultParameterValuesHC -Path 'Test-Function'

Now to get the default values in the function arguments one can use AST:

Function Get-DefaultParameterValuesHC {
    [OutputType([hashtable])]
    Param (
        [Parameter(Mandatory)]$Path
    )
    $ast = (Get-Command $Path).ScriptBlock.Ast
        
    $selectParams = @{
        Property = @{ 
            Name       = 'Name'; 
            Expression = { $_.Name.VariablePath.UserPath } 
        },
        @{ 
            Name       = 'Value'; 
            Expression = { $_.DefaultValue.Extent.Text -replace "`"|'" }
        }
    }
        
    $result = @{ }

    $defaultValueParameters = @($ast.FindAll( { 
                $args[0] -is [System.Management.Automation.Language.ParameterAst] }
            , $true) | 
        Where-Object { $_.DefaultValue } | 
        Select-Object @selectParams)
            
    foreach ($d in $defaultValueParameters) {
        $result[$d.Name] = foreach ($value in $d.Value) {
            $ExecutionContext.InvokeCommand.ExpandString($value)
        }
    }
    $result
}

The issue here is that the argument for $ComputerNames is read as a string while it is actually an array of string.

Is there a way that PowerShell can covnert a string to an array? Or even better, read the value correctly in the first place?

Because it returns the string @($env:COMPUTERNAME, 'PC2') and not 2 separate items as expected. — DarkLite1
– DarkLite1, Commented Oct 26, 2020 at 8:19

iRon · Accepted Answer · 2020-10-26 13:29:04Z

1

You need to look deeper into the AST structure...
I recommend you to play around with this PowerShell: AST Explorer GUI:

For your specific example:

Function Test-Function {
    Param (
        [String[]]$ComputerNames = @($env:COMPUTERNAME, 'PC2'),
        [String]$PaperSize = 'A4'
    )
}

$FunctionDefinitionAst = (Get-Command 'Test-Function').ScriptBlock.Ast
$Body = $FunctionDefinitionAst.Body
$ParamBlock = $Body.ParamBlock
$CNParameter = $ParamBlock.Parameters | Where-Object { $_.Name.VariablePath.UserPath -eq 'ComputerNames' }
$DefaultValue = $CNParameter.DefaultValue
$DefaultValue.SubExpression.Statements.PipelineElements.Expression.Elements

VariablePath : env:COMPUTERNAME
Splatted     : False
StaticType   : System.Object
Extent       : $env:COMPUTERNAME
Parent       : $env:COMPUTERNAME, 'PC2'

StringConstantType : SingleQuoted
Value              : PC2
StaticType         : System.String
Extent             : 'PC2'
Parent             : $env:COMPUTERNAME, 'PC2'

edited Oct 26, 2020 at 13:29

answered Oct 26, 2020 at 11:59

iRon

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1 Comment

DarkLite1 Over a year ago

That's a lot cleaner then what I did. Thank you iRon appreciate the help.

DarkLite1 · Accepted Answer · 2020-10-26 08:22:28Z

It's a bit of a hackish solution but this is what I came up with to solve the issue of not returning an array of string:

Function Get-DefaultParameterValuesHC {
    Param (
        [Parameter(Mandatory)]$Path
    )
    $ast = (Get-Command $Path).ScriptBlock.Ast
        
    $selectParams = @{
        Property = @{ 
            Name       = 'Name'; 
            Expression = { $_.Name.VariablePath.UserPath } 
        },
        @{ 
            Name       = 'Value'; 
            Expression = { 
                if ($_.DefaultValue.StaticType.BaseType.Name -eq 'Array') {
                    $_.DefaultValue.SubExpression.Extent.Text -split ',' | 
                    ForEach-Object { $_.trim() -replace "`"|'" }
                }
                else {
                    $_.DefaultValue.Extent.Text -replace "`"|'" 
                }
            }
        }
    }
        
    $result = @{ }

    $defaultValueParameters = @($ast.FindAll( { 
                $args[0] -is [System.Management.Automation.Language.ParameterAst] }
            , $true) | 
        Where-Object { $_.DefaultValue } | 
        Select-Object @selectParams)
            
    foreach ($d in $defaultValueParameters) {
        $result[$d.Name] = foreach ($value in $d.Value) {
            $ExecutionContext.InvokeCommand.ExpandString($value)
        }
    }
    $result
}

marsze · Accepted Answer · 2020-10-26 13:37:39Z

ExpandPath will only expand variables inside strings. To get the actual values (and not just the definition) you could use Invoke-Expression:

function Get-DefaultParameterValuesHC {
    [OutputType([hashtable])]
    Param (
        [Parameter(Mandatory)]$Path
    )
    $result = @{ }
    (Get-Command $Path).ScriptBlock.Ast.Body.ParamBlock.Parameters | where {$_.DefaultValue} | foreach {
        $result[$_.Name.VariablePath.UserPath] = Invoke-Expression $_.DefaultValue.Extent.Text
    }
    $result
}

NOTE: This will actually invoke the default declaration, so any logic inside that expression will be run, just as when running the function. For example, a default value of $Parameter = (Get-Date) will always invoke Get-Date.

It would be preferable to create a function, that only returns the default declarations, and let the user decide to invoke the expression or not:

function Get-DefaultParameterDeclarations {
    Param (
        [Parameter(Mandatory, Position = 0)]
        [string]$CommandName
    )
    (Get-Command $CommandName).ScriptBlock.Ast.Body.ParamBlock.Parameters | where {$_.DefaultValue} |
      foreach {
        [PSCustomObject]@{
            Name = $_.Name.VariablePath.UserPath
            Expression = $_.DefaultValue.Extent.Text
        }
    }
}

# get the declarations and (optionally) invoke the expressions:
Get-DefaultParameterDeclarations 'Test-Function' |
    select Name, @{n="DefaultValue"; e={Invoke-Expression $_.Expression}}

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