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When writing code in PowerShell, you'll occasionally have the need to pass all parameters from a function directly to another function reference inside. This is common in instances where you might have similar functions with similar parameters but not all parameters are the same.
For example, I might have two functions that look something like this:
function Get-Something
{
[CmdletBinding()]
param
(
[Parameter(Mandatory)]
[string]$Thing1,
[Parameter(Mandatory)]
[string]$Thing2,
[Parameter()]
[string]$Thing3 = 'adefaultvalue'
)
Set-Something –Thing1 $Thing1 –Thing2 $Thing2 –Thing3 $Thing3
}
function Set-Something
{
[CmdletBinding()]
param
(
[Parameter(Mandatory)]
[string]$Thing1,
[Parameter(Mandatory)]
[string]$Thing2,
[Parameter(Mandatory)]
[string]$Thing3
)
Write-Host "Setting something with Thing1: $($Thing1) – Thing2: $($Thing2) – Thing3: $($Thing3)"
}
I can run this, and it performs as you'd expect.
Successful functions
All parameters of each function represent the same thing in both. Notice that the Set-Something function is referenced inside of the Get-Something function. Inside here, I am passing all of the variables (both bound and default) from Get-Something's parameters to the Set-Something function. This works fine, but it's not efficient coding. Savvy PowerShellers would know that parameters passed to a function are available inside of the $PSBoundParameters variable. If all of the parameters are inside of this variable already, there's no sense redefining each parameter and passing it to Set-Something. Instead, we can use splatting and do the following:
function Get-Something
{
[CmdletBinding()]
param
(
[Parameter(Mandatory)]
[string]$Thing1,
[Parameter(Mandatory)]
[string]$Thing2,
[Parameter()]
[string]$Thing3 = 'adefaultvalue'
)
Set-Something @PSBoundParameters
}
Since Thing3 is a default parameter, when you call Get-Something, you'd expect the values of Thing1, Thing2, and Thing3 to be passed to Set-Something.
Get-Something –Thing1 'hello' –Thing2 'world'
Let's see what happens.
The values of Thing1, Thing2, and Thing3 to be passed to Set-Something
Set-Something is prompting for a value for Thing3. Why? Since I set the default value of Thing3 on Get-Something's parameter, it should have worked, right? No. The reason is that $PSBoundParameters includes only parameters that were bound, meaning it does not contain any parameter arguments defined with a default value.
I need to figure out a way to get the value of Get-Something's Thing3 variable other than explicitly calling $Thing3. We can do this by using the Abstract Syntax Tree (AST). The AST is a great way to gather all kinds of information about your PowerShell scripts and functions. By using the AST and looking for all of the System.Management.Automation.Language.ParameterAst objects inside of this function, we can discover this information.
To find that default value, I'll first need to convert the Get-Something function into an AST block. I can do this by calling Get-Command.
$ast = (Get-Command 'Get-Something').ScriptBlock.Ast
Once I have the function that represents as an AST block, I can now search for various components inside using the FindAll () method.
$ast.FindAll({ $args[0] -is [System.Management.Automation.Language.ParameterAst] }, $true)
Searching for various components inside using the FindAll () method
You can see that this returns a lot of things other than the parameter name and default value. We'll need to pare this down a little bit to get something more usable. I'd like to get an output that includes just all of the parameters, with a default value showing their name and value only.
I can do that by digging into the objects a little bit and creating my own calculated property with Select-Object.
$myProperties = @{ n = 'Name'; e = { $_.Name.VariablePath.UserPath } },
@{ n = 'Value'; e = { $_.DefaultValue.Extent.Text -replace "`"|'" } }
$params = $ast.FindAll({ $args[0] -is [System.Management.Automation.Language.ParameterAst] }, $true) | where { $_.DefaultValue } | select $myProperties
This will then give me an output that's much cleaner.
Cleaner output
Great! We now have a way to find all of the bound and default values for the Get-Something parameter. Let's now take the
function Get-Something
{
[CmdletBinding()]
param
(
[Parameter(Mandatory)]
[string]$Thing1,
[Parameter(Mandatory)]
[string]$Thing2,
[Parameter()]
[string]$Thing3 = 'adefaultvalue'
)
$ast = (Get-Command 'Get-Something').ScriptBlock.Ast
$myProperties = @{ n = 'Name'; e = { $_.Name.VariablePath.UserPath } },
@{ n = 'Value'; e = { $_.DefaultValue.Extent.Text -replace "`"|'" } }
$defaulParams = @{}
@($ast.FindAll({ $args[0] -is
[System.Management.Automation.Language.ParameterAst] }, $true) | where { $_.DefaultValue } | select $myProperties).foreach({
$defaultParams[$_.Name] = $_.Value
})
$setSomethingParams = $PSBoundParameters + $defaultParams
Set-Something @setSomethingParams
}
I can now run Get-Something again, and I now get the default value as well.
Running Get Something again and getting the default value
You might be thinking that this would be overkill, and you'd be right for this simple example. But if you find yourself working with functions that contains dozens of parameters with default values, this method will take a ton of time.
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If you'd like to save some lines, I've wrapped this up into a function called Get-DefaultFunctionParameter, which you can download at GitHub and reuse.
