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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