PowerShell function skips over switch statement

Question

I have a Hyper-V test environment on my work PC where I use a small PowerShell function for quickly creating new VMs from exported ones that I use as templates. But when I added a switch statement to the function in order to enhance template selection I discovered that PowerShell ignores the switch statement completely.

By stripping down the function I found out that the problem arises from a parameter named $Switch (for attaching the new VM to a virtual switch).

Demonstration:

function foo {
  Param(
    [String] $Switch
  )

  'start'
  switch ($true) {
    default { 'should display' }
  }
  'end'
}

Invoking the above function (with or without argument) produces the following output:

start
end

when it should produce:

start
should display
end

Renaming the parameter resolves the issue, so I'm going to do that, but even so I'd like to understand what is going wrong here.

Answer 1

You experience this issue because $Switch is actually one of PowerShell's automatic variables. Getting weird behavior from using the names of automatic variables for parameters is a common PowerShell gotcha.

From about_Switch:

The switch statement can use the $_ and $switch automatic variables. For more information, see about_Automatic_Variables.

and from about_Automatic_Variables:

$switch
Contains the enumerator not the resulting values of a Switch statement. The $switch variable exists only while the Switch statement is running; it's deleted when the switch statement completes execution. For more information, see about_Switch.

The solution to this problem is to avoid using automatic variable names for parameters. Rename the parameter (e.g. to $VSwitch) and the problem will disappear. Optionally you can define an alias for the parameter

function foo {
  Param(
    [Alias('Switch')]
    [String] $VSwitch
  )
  #...
}

so that you retain the ability to invoke the function like this

foo -Switch 'something'

That would save you the trouble of having to refactor all code that uses the function.