Why this code passing switch parameters to a PowerShell function fails - powershell

Question

This question is about passing switch parameters. Let's see the code. I have this PowerShell 3.0 function:
#test1.ps1
param(
[switch] $param1 = $false
)
Write-Host "param1: $($param1.IsPresent)"
Write-Host
I have this main PowerShell function that invokes test.ps1 in four different ways:
#Test0.ps1
cls
$param1 = $True
# 1
.\test1.ps1 -param1
# 2
.\test1.ps1 -param1:$true
# 3
$potato = "-param1:`$$($param1)"
Write-Host "Parameter value: $potato"
.\test1.ps1 $potato
# 4
$command = ".\test1.ps1 -param1:`$$($param1)"
Write-Host "Command: $command"
iex $command
exit
Why is the 3rd way of doing it failing? I know I can do 4th way but I would love to understand why 3rd is failing.
Here is the output. As result all the parameters should be True but third one is False...
param1: True
param1: True
Parameter value: -param1:$True
param1: False
Command: .\test1.ps1 -param1:$True
param1: True

Answer 1

What happens is that:
The parser looks at the provided argument: "-param1:$true"
Fails to bind it to parameter param1, since the value you provided is a string, not a switch/bool
No specific parameter type is required at position 0, argument is ignored
If you make param1 positional, you can see how PowerShell fails to bind it properly:
function test-parambinding {param([Parameter(Position=0)][switch]$param1);$param1.IsPresent}
test-parambinding "-param1:`$true"
You'll see a ParameterArgumentTransformationException thrown before anything else happens

Answer 2

Mathias explains why your 3rd way of passing the parameter fails, but there is another way to pass parameters that lets you do roughly what you seem to be attempting here.
I've used a function here as it's a bit less to type when calling it, but your script file will work just the same:
PS C:\> function test1() {
param(
[switch] $param1 = $false
)
Write-Host "param1: $($param1.IsPresent)"
Write-Host
}
PS C:\> $param1 = $True
PS C:\> $potato = #{'param1'=$param1}
PS C:\> $potato
Name Value
---- -----
param1 True
PS C:\> test1 #potato
param1: True
So, instead of passing the argument and value as a single string, create a hash with the argument name as the key and the argument as the value. Then call the function or script using the # splatting operator. (See help about_Splatting for more detail).