powershell - How do I pass a scriptblock as one of the parameters in start-job

Question

I'm trying to create a background job, which executes a scriptblock. I need to pass this scriptblock in as a parameter, but I can't seem to get the syntax to work. The scriptblock is being converted to a string somewhere along the way.

It works fine when I pass the script block to a local function, but not through start-job

The following syntax works:

function LocalFunction
{
    param (
        [parameter(Mandatory=$true)]
        [ScriptBlock]$ScriptBlock
    )

    &$ScriptBlock | % { echo "got $_" }
}

LocalFunction -ScriptBlock { echo "hello" }

This outputs "got hello" as expected.

But the following fails:

$job = start-job -argumentlist { echo "hello" } -scriptblock {
    param (
        [parameter(Mandatory=$true)]
        [ScriptBlock]$ScriptBlock
    )
    &$ScriptBlock | % { echo "got $_" }
}
start-sleep -s 1
receive-job $job

The error it returns is

Receive-Job : Cannot process argument transformation on parameter 'ScriptBlock'. Cannot convert the " echo "hello" " value of type "System.String" to type "System.Management.Automation.ScriptBlock".

So if I'm reading the error right, it appears that -argumentlist is somehow forcing its arguments into strings.

Any powershell gurus know how to make this syntax work?

Thanks in advance,

Ben

Answer 1

Here's one way to solve this, pass the scriptblock code as a string, then create a scriptblock from the string inside the job and execute it

Start-Job -ArgumentList "write-host hello"  -scriptblock {

    param (
        [parameter(Mandatory=$true)][string]$ScriptBlock
    )

    & ([scriptblock]::Create($ScriptBlock))

} | Wait-Job | Receive-Job