PowerShell function declaration pitfall

Recently I had to write some scripts in PowerShell. Because I try to follow good software development practices I decided to split script logic into functions. First I defined a few functions that taken or returned single value:

function printArg($arg1) {
    write-host "printArg: arg1=$arg1";
}
    
function transformArg($n) {
    return $n*$n
}

Of course they worked brilliantly:

> printArg(1)
printArg: arg1=1
    
> printArg("foo")
printArg: arg1=foo
    
> write-host $(transformArg(3))
9

But then I needed to declare function that takes three arguments, I started with code:

function manyArgs($arg1, $arg2, $arg3) {
    write-host "manyArgs: arg1=$arg1, arg2=$arg2, arg3=$arg3"
}

Unfortunatelly it didn’t work as expected:

> manyArgs(1, 2, 3)
manyArgs: arg1=1 2 3, arg2=, arg3=

After a bit of research it turned out that PowerShell uses space as argument separator. So to pass three arguments we need to write:

> manyArgs 1 2 3
manyArgs: arg1=1, arg2=2, arg3=3
    
# or using more verbose syntax:
> manyArgs -arg1 1 -arg2 2 -arg3 3
manyArgs: arg1=1, arg2=2, arg3=3

This also works with single argument functions:

> printArg 1
printArg: arg1=1
    
> printArg -arg1 1
printArg: arg1=1

Before we end let’s find out what exactly is (1, 2, 3) and why calling printArg(1) works. It turns out that (a, b, ..., z) expression represents array of objects in PowerShell:

> (1, 2, 3).getType().name
Object[]

And (x) expression represents just x value :

> (1).getType().name
Int32

and thats explain outputs of printArg(1) and manyArgs(1, 2, 3) command.

This behaviour of PowerShell may be surprising for C# programmers, I hope this post will help you remember about this problem.

marcin-chwedczuk

A Programmer, A Geek, A Human