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Overwriting a PowerShell Constant

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I have always said that one of the greatest benefit to teaching PowerShell (and Windows) is that different people bring different ideas to the table.  Things get fun for me when someone looks at what we are doing from a different angle and asked an interesting question.

This week’s class in Fort Wayne produced one of those questions.  We were looking at some of the options that are available to use with creating variables with the New-Variable cmdlet.  In particular, we were looking at constants.  Let’s build one.

PS C:\> New-Variable -Name Test1 -Value ([Bool]$True) -Option Constant

Now let’s take a look at the variable object.
Name        : Test1
Description :
Value       : True
Visibility  : Public
Module      :
ModuleName  :
Options     : Constant
Attributes  : {}


We can see from the Options property that we have created a constant.  We are going to attempt to change that value of this constant to FALSE.

PS C:\> Set-Variable -Name Test1 -Value $false
Set-Variable : Cannot overwrite variable Test1 because it is read-only or constant.
At line:1 char:1
+ Set-Variable -Name Test1 -Value $false
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : WriteError: (Test1:String) [Set-Variable], SessionStateU
   nauthorizedAccessException
    + FullyQualifiedErrorId : VariableNotWritable,Microsoft.PowerShell.Commands.SetVar
   iableCommand

This is what we expected.  By definition, a constant cannot be changed.  We also attempted to change it with the –Force parameter.

PS C:\> Set-Variable -Name Test1 -Value $false -Force
Set-Variable : Cannot overwrite variable Test1 because it is read-only or constant.
At line:1 char:1
+ Set-Variable -Name Test1 -Value $false -Force
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : WriteError: (Test1:String) [Set-Variable], SessionStateU
   nauthorizedAccessException
    + FullyQualifiedErrorId : VariableNotWritable,Microsoft.PowerShell.Commands.SetVar
   iableCommand

Again, the expected results.  Well, this is where that question comes into play.  What if you re-cast the variable?  OK, let’s give this a try.

PS C:\> [Bool]$Test1 = $False

PS C:\> $Test1
False

Wait… What?  You cannot even get rid of a constant with Remove-Variable but here we changed it.  OK, did we really change the value or did it delete the variable and then recreate it?  Here is another test.

PS C:\> Set-Variable -Name Test3 -Value ([Bool]$True) -Option Constant -Description "This is a test"


Here we included a description which you can see in the variable objects properties.

Name        : Test3
Description : This is a test
Value       : True
Visibility  : Public
Module      :
ModuleName  :
Options     : Constant
Attributes  : {}

We are going to change this variable using the same successful method from above.

PS C:\> [Bool]$Test3 = $False

PS C:\> $Test3
False


And now let’s look at the properties to see if the description is still there.
PS C:\> Get-Variable -Name Test3 | Select-Object -Property *


PSPath        : Microsoft.PowerShell.Core\Variable::Test3
PSDrive       : Variable
PSProvider    : Microsoft.PowerShell.Core\Variable
PSIsContainer : False
Name          : Test3
Description   : This is a test
Value         : False
Visibility    : Public
Module        :
ModuleName    :
Options       : Constant
Attributes    : {System.Management.Automation.ArgumentTypeConverterAttribute}



The description is still there.  So, I guess there is a way to change the value of a constant without restarting PowerShell
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