PowerShell functions argument catching

PowerShell is not very object oriented. The support for creating classes is implemented through the Add-Type cmdlet that takes C# code. Here I show how to use Add-Type to define and expose classes that use any part of the .NET Framework. These classes can then be instantiated using the New-Object cmdlet.

Functions are defined using the function keyword. This code defines a function that doesn’t require any arguments:

#Define a function.
function DoSomething()
{
   Write-Output "Hello!"
}

#Call a function.
DoSomething

When defining parameters, you can choose what parameters that need to be strongly typed by adding the desired type name in front of the parameter. Here I have 3 calls to the TwoParams function. The first parameter can be anything, but the second must be and int. Both call 1 and call 2 will run, becase $a can accept a string or an int. Call 3 will fail because $b cannot be a string.

function TwoParams($a, [int]$b)
{
	$b = $b + 1
	Write-Output "a=$a"
	Write-Output "b=$b"
}

#Two correct calls.
Write-Output "Call 1"
TwoParams "Hello!" 10

Write-Output "Call 2"
TwoParams 20 20

Write-Output "Call 3 will fail!"
TwoParams 30 "Hello!"

Default values can be provided, so that omitted arguments still will have a value. In this case, it is assumed that the values 1 and 2 is passed to the function. All parameters are passed by value.

function TestingDefaults($a=1, $b=2)
{
	Write-Output "$a $b"
}

TestingDefaults

Now, let’s look at the strange things. All arguments that are sent in, but not declared in the function head, will be captured by the $args array. In this case, $args in SomeFunction will contain 4 elements, because 4 arguments are sent:

function SomeFunction()
{
	$NumberOfArguments=$args.length
	Write-Output "You gave me $NumberOfArguments arguments."
}

SomeFunction "10" "Hej" 5 10

But here, still 4 arguments are sent, but the first argument is declared, so that only 3 arguments are catched by the $args array:

function SomeFunction($x)
{
	$NumberOfArguments=$args.length
	Write-Output "You gave me $NumberOfArguments arguments."
}

SomeFunction "10" "Hej" 5 10