Powershell: How do I pass variables to switch parameters when invoking powershell at the command line?

Question

Normally, if you want to defer the specification of a switch parameter to some variable, you can pass an expression to the switch parameter, as seen with the WhatIf parameter.

test.ps1

param ( [string] $source, [string] $dest, [switch] $test )
Copy-Item -Path $source -Destination $dest -WhatIf:$test

This allows you great flexibility when working with switches. However, when you call powershell with cmd.exe or something, you wind up with something like this:

D:\test>powershell -file test.ps1 -source test.ps1 -dest test.copy.ps1 -test:$true

D:\test\test.ps1 : Cannot process argument transformation on
parameter 'test'. Cannot convert value "System.String" to type "System.Manageme
nt.Automation.SwitchParameter", parameters of this type only accept booleans or
 numbers, use $true, $false, 1 or 0 instead.
At line:0 char:1
+  <<<<
    + CategoryInfo          : InvalidData: (:) [test.ps1], ParentContainsError
   RecordException
    + FullyQualifiedErrorId : ParameterArgumentTransformationError,test.ps1

However, the same result appears when passing -test:true, and -test:1. Why doesn't this work? Shouldn't Powershell's type conversion system automatically recognize these strings as being convertible to bool or switch, and convert them?

Does this mean that when calling powershell scripts from some other system (such as a build system) it's necessary to construct complex flow control structures to determine whether or not to include a switch in the command string, or omit it? This seems tedious and error prone, which leads me to believe it's not the case.

Moderato

Answer 1

This behaviour has been filed as a bug on connect. This is a workaround:

powershell ./test.ps1 -source test.ps1 -dest test.copy.ps1 -test:$true

Angiosperm

Notice this answer is obsolete -- Microsoft Connect is retired and there's no way to tell the status of the bug from the link. – Downpipe

Answer 2

Use the IsPresent property of the switch. Example:

function test-switch{
param([switch]$test)
  function inner{
    param([switch]$inner_test)
    write-host $inner_test
  }
  inner -inner_test:$test.IsPresent
}
test-switch -test:$true
test-switch -test
test-switch -test:$false

True
True
False

BTW, I used functions rather than a script so it would be easier to test.

Rew

That's actually what I tried in the first example--the problem seems to be that powershell doesn't evaluate the parameter as an expression when the script is invoked from cmd.exe, but rather that it takes "$true" as an opaque string. – Moderato