calling Invoke-Expression with Parameters in Powershell

Question

I've written a powershell module in c# that has a bunch of cmdlets like

Add-VM

The cmdlets reach out to an API and pull data back.

but for the sake of uniformity with the ssh CLI of the product, i've written a function called newtask that accepts 'addvm' as an argument and $args.

for example

newtask addvm -id 12345

I then invoke Add-VM and pass $args as a string like so

Invoke-Expression Add-VM $argstr

The problem is that Add-VM throws an error that it cannot find a positional parameter that accepts argument System.Object[]

A positional parameter cannot be found that accepts argument 'System.Object[]'

While I could easily alias 'addvm' to 'Add-VM', i'm trying to maintain uniformity with the ssh CLI so that new users can quickly start utilizing this module.

I figured that sending a string like '-id 12345' would suffice but it's not. Does the pscmdlet expect to receive something else?

Thanks in advance.

I'm pretty sure that error is from Invoke-Expression not Add-VM and you just need quotes around the argument: Invoke-Expression "Add-VM $argstr". — Mike Zboray
– Mike Zboray, Commented Oct 28, 2014 at 18:45
As an aside, why would I not just use Add-VM directly? This way just seems to add more typing. — Mike Zboray
– Mike Zboray, Commented Oct 28, 2014 at 18:47
@mikez while i agree with you, this module is for a product and the current users of a product are used to a certain syntax like i mentioned in the question. In the case of 'addvm', Add-VM makes sense but Delete is not a powershell verb so i had to use remove-vm for 'deletevm'. I'm trying to keep it as close to the original ssh shell on the product so that the transition is seamless. — anoopb
– anoopb, Commented Oct 28, 2014 at 19:42
If you want to use invocation as an abstraction layer you may want to use Invoke-Command -ScriptBlock { ... $args } -ArgumentList ... rather than Invoke-Expression "... $argstr". — Ansgar Wiechers
– Ansgar Wiechers, Commented Oct 28, 2014 at 20:07

getSurreal · Accepted Answer · 2016-08-15 16:35:58Z

12

I know this is a little old now, but I was having a similar issue and a co-worker showed me that escaping $argstr prevents the object from getting converted to a string.

Invoke-Expression "Add-VM `$argstr"

edited Aug 15, 2016 at 16:35

answered Aug 15, 2016 at 16:20

getSurreal

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Mike Zboray · Accepted Answer · 2014-10-28 20:29:47Z

5

That error is from Invoke-Expression not Add-VM and you just need quotes around the argument:

Invoke-Expression "Add-VM $argstr"

This has the drawback of forcing all objects into string format. This might be acceptable for simple types like ints and strings but if you want to pass through a more complex object it won't work. An alternative would be to splat the arguments with @args but I don't think you can do this through Invoke-Expression or Invoke-Command. You need to directly call the cmdlet:

function newtask {
    params([string]$command)

    switch ($command) {
        "addvm" { Add-VM @args }
        "deletevm" { Remove-VM @args }
    }
}

answered Oct 28, 2014 at 20:29

Mike Zboray

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1 Comment

anoopb Over a year ago

This is great. Thanks for posting. Another option, which i'm using, is an array and simply check to see if $command is an element in $cmdlist.

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