Start-Job - Positional order of arguments - powershell

Question

I'm trying to understand the correct order of arguments when using Start-Job. What is the correct way to supply parameters to a PowerShell job?
I would expect this to print hello world, but it prints world hello.
Is Param() or -ArgumentList the issue here?
$foo = "hello"
$bar = "world"
$job = Start-Job -ScriptBlock {
Param(
$foo,
$bar
)
Write-Host $foo
Write-Host $bar
} -ArgumentList $bar, $foo
Receive-Job $job
Output:
world
hello

Answer 1

The argument of the parameter -ArgumentList is an array, whose values are passed to parameters defined inside the scriptblock in positional order. You're confused about the result you're getting, because you apparently expected your global variables to be mapped to the parameter names you defined in your scriptblock. That is not how this works.
To maybe illustrate a bit better what is happening in your example let's use distinct variable names in the scriptblock and global scope:
$a = "hello"
$b = "world"
$job = Start-Job -ScriptBlock {
Param(
$c,
$d
)
Write-Host $c
Write-Host $d
} -ArgumentList $b, $a
Essentially, the names of the parameters have nothing to do with the names of the variables in the global scope.
You're switching the values when you're passing $b, $a to the scriptblock instead of $a, $b, hence the value of $b is passed to $c and the value of $a is passed to $d.
Normally one would use splatting for mapping values to specific named parameters. However, that won't work here, since -ArgumentList expects an array of values, not a hashtable. If the difference between positional and named parameters is not clear to you please have a look at the documentation.
What you can do if you want to use the same variable names inside and outside the scriptblock is use the using: scope qualifier instead of passing the variables as arguments:
$a = "hello"
$b = "world"
$job = Start-Job -ScriptBlock {
Write-Host $using:a
Write-Host $using:b
}