How and Why to Use Splatting (passing [switch] parameters)
On our internal discussion list, someone just asked about how to pass switch parameters from one function to another. This person asked this as "Why can't [switch] parameters be passed as parameters?".
My short answer was simple: "They Can Be"
Answering this question is a great way to illustrate the value of a PowerShell V2 feature called Splatting. Splatting is two things, it's a way to get all of the parameters passed to a function automatically (they're in a variable, $psBoundParameters), and it's a way to take a list or dictionary of arguments and pass them on to the next function below.
The mistake of trying to pass along a switch parameter is a pretty easy one to make. [switch] will let you specify the parameter by just passing -ParameterName instead of -ParameterName True.
When you try to pass on a value to another underlying function, -SwitchParameterInOtherFunction $switchParameterInThis function will set the switch to true in the underlying function, and then pass a positional parameter with the value of the switch parameter.
The V1 fix for this is to put a : between the parameter name and its value. The V2 fix for this is to simply use splatting. The important line below is in bold and italics.
| V2: | V1: |
| function test2($x, $y, [switch]$passThru) {
"In Test 2"
"x:"
$x
"y:"
$y
"passThru:"
$passThru
} function test1($x, $y, [switch]$passThru) {
"In Test 1"
Test2 @psBoundParameters
} test1 "a" "b" -passThru | function test2($x, $y, [switch]$passThru) {
"In Test 2"
"x:"
$x
"y:"
$y
"passThru:"
$passThru
} function test1($x, $y, [switch]$passThru) {
"In Test 1"
Test2 -x:$x -y:$y -passThru:$passThru
} test1 "a" "b" -passThru |
So why use splatting? Well, here was my answer to the question on our PowerShell Discussions alias:
Notice that with @psBoundParameters, I don’t have to specify the parameters on by one. This is both more convenient (less typing) and less error prone (less chance of forgetting a parameter or adding a typo).
To see more powerful examples of splatting, you can check out Get-CommandPlugin and the CodeDownloader module.
Hope this Helps,
James Brundage [MSFT]
