Function pointer in Powershell

Question

Consider the following function

Function IfFunctionExistsExecute
{
    param ([parameter(Mandatory=$true)][string]$func)
    begin 
    {
        # ...
    }
    process
    {
        if(Get-Command $func -ea SilentlyContinue)
        {
            & $func # the amperersand invokes the function instead of just printing the variable
        }
        else
        {
            # ignore
        }       
    }
    end
    {
        # ...
    }
}

Usage:

Function Foo { "In Foo" }
IfFunctionExistsExecute Foo

This works.

However this doesn't work:

Function Foo($someParam) 
{ 
     "In Foo"
     $someParam
}

IfFunctionExistsExecute Foo "beer"

However this gives me the ugly error:

IfFunctionExistsExecute : A positional parameter cannot be found that accepts argument 'beer'.
At C:\PSTests\Test.ps1:11 char:24
+ IfFunctionExistsExecute <<<<  Foo "beer"
    + CategoryInfo          : InvalidArgument: (:) [IfFunctionExistsExecute], ParameterBindingException
    + FullyQualifiedErrorId : PositionalParameterNotFound,IfFunctionExistsExecute

How can I do this in PS?

Patrick Cuff · Accepted Answer · 2012-07-06 17:30:38Z

2

Try creating an optional parameter on the function your calling, and on your IfFunctionExistsExecute function; something like this:

Function IfFunctionExistsExecute
{
    param ([parameter(Mandatory=$true)][string]$func, [string]$myArgs)
        if(Get-Command $func -ea SilentlyContinue)
        {
            & $func $myArgs  # the amperersand invokes the function instead of just printing the variable
        }
        else
        {
            # ignore
        }       
}

Function Foo
{ 
    param ([parameter(Mandatory=$false)][string]$someParam)
    "In Foo" 
    $someParam
}

IfFunctionExistsExecute Foo
IfFunctionExistsExecute Foo "beer"

For me this gives:

C:\test>powershell .\test.ps1
In Foo

In Foo
beer

C:\test>

edited Jul 6, 2012 at 17:30

answered Jul 6, 2012 at 12:11

Patrick Cuff

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2 Comments

Anemoia Over a year ago

Seems that the issue was that my module wasn't being reloaded when I edited it :(

klumsy Over a year ago

don't use $args as your own parameter name , as that is a built in automatic parameter name.

Joey · Accepted Answer · 2012-07-06 11:35:27Z

1

Maybe you should pass arguments to the called function as well:

$arguments = $args[1..($args.Length-1)]
& $func @arguments

answered Jul 6, 2012 at 11:35

Joey

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2 Comments

Anemoia Over a year ago

Nope, doesn't do the trick, same error... I even tried to add the $args to the param list of the IfFunctionExistsExecute method.

Anemoia Over a year ago

Ok, it works half, the @ thingy (whatever that does) passes it in as comma seperated, like this: Foo b,e,e,r. Which is not what I want.

groser · Accepted Answer · 2023-04-04 17:17:33Z

I wrote my solution. Function named by powershell's style: Verb-Noun. It supports named parameters and remaining arguments.

function Invoke-FunctionIfExist {
    [CmdletBinding()]
    param (
        # Function name
        [Parameter(Mandatory, Position=0)]
        [string]
        $Name,
        # Hashtable with named arguments
        [Parameter()]
        [hashtable]
        $Parameters = @{},
        # Rest of arguments
        [Parameter(ValueFromRemainingArguments)]
        [object[]]
        $Arguments = @()
    )

    process {
        if (Get-Command $Name -ErrorAction SilentlyContinue) {
            & $Name @NamedArguments @Arguments
        }
    }
}

Usage example:

PS> Invoke-FunctionIfExist Foo
In Foo
PS> Invoke-FunctionIfExist Foo beer
In Foo
beer
PS> Invoke-FunctionIfExist Foo -Parameters @{someParam='beer'}
In Foo
beer

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