Does PowerShell support constants?

I would like to declare some integer constants in PowerShell.

Is there any good way to do that?

Use

Set-Variable test -Option Constant -Value 100

or

Set-Variable test -Option ReadOnly -Value 100

The difference between "Constant" and "ReadOnly" is that a read-only variable can be removed (and then re-created) via

Remove-Variable test -Force

whereas a constant variable can't be removed (even with -Force).

See this TechNet article for more details.

Here is a solution for defining a constant like this:

const myConst = 42

Solution taken from http://poshcode.org/4063

    function Set-Constant {
  <#
    .SYNOPSIS
        Creates constants.
    .DESCRIPTION
        This function can help you to create constants so easy as it possible.
        It works as keyword 'const' as such as in C#.
    .EXAMPLE
        PS C:\> Set-Constant a = 10
        PS C:\> $a += 13
 
        There is a integer constant declaration, so the second line return
        error.
    .EXAMPLE
        PS C:\> const str = "this is a constant string"
 
        You also can use word 'const' for constant declaration. There is a
        string constant named '$str' in this example.
    .LINK
        Set-Variable
        About_Functions_Advanced_Parameters
  #>
  [CmdletBinding()]
  param(
    [Parameter(Mandatory=$true, Position=0)]
    [string][ValidateNotNullOrEmpty()]$Name,
 
    [Parameter(Mandatory=$true, Position=1)]
    [char][ValidateSet("=")]$Link,
 
    [Parameter(Mandatory=$true, Position=2)]
    [object][ValidateNotNullOrEmpty()]$Mean,
 
    [Parameter(Mandatory=$false)]
    [string]$Surround = "script"
  )
 
  Set-Variable -n $name -val $mean -opt Constant -s $surround
}
 
Set-Alias const Set-Constant

Use -option Constant with the Set-Variable cmdlet:

Set-Variable myvar -option Constant -value 100

Now $myvar has a constant value of 100 and cannot be modified.

To use a specific type of value, say Int64, you can explicitly cast the value used in set-variable.

For instance:

set-variable -name test -value ([int64]100) -option Constant

To check,

$test | gm

And you'll see that it is an Int64 (rather than Int32, which would be normal for the value 100).

I really like the syntactic sugar that rob's answer provides:

const myConst = 42

Unfortunately his solution doesn't work as expected when you define the Set-Constant function in a module. When called from outside the module, it will create a constant in the module scope, where Set-Constant is defined, instead of the caller's scope. This makes the constant invisible to the caller.

The following modified function fixes this problem. The solution is based on this answer to the question "Is there any way for a powershell module to get at its caller's scope?".

function Set-Constant {
    <#
    .SYNOPSIS
        Creates constants.
    .DESCRIPTION
        This function can help you to create constants so easy as it possible.
        It works as keyword 'const' as such as in C#.
    .EXAMPLE
        PS C:\> Set-Constant a = 10
        PS C:\> $a += 13

        There is a integer constant declaration, so the second line return
        error.
    .EXAMPLE
        PS C:\> const str = "this is a constant string"

        You also can use word 'const' for constant declaration. There is a
        string constant named '$str' in this example.
    .LINK
        Set-Variable
        About_Functions_Advanced_Parameters
    #>
    [CmdletBinding()]
    param(
        [Parameter(Mandatory=$true, Position=0)] [string] $Name,
        [Parameter(Mandatory=$true, Position=1)] [char] [ValidateSet("=")] $Link,
        [Parameter(Mandatory=$true, Position=2)] [object] $Value
    )

    $var = New-Object System.Management.Automation.PSVariable -ArgumentList @(
        $Name, $Value, [System.Management.Automation.ScopedItemOptions]::Constant
    )
    
    $PSCmdlet.SessionState.PSVariable.Set( $var )
}

Set-Alias const Set-Constant

Notes:

• The function only works, when called from outside the module, where it is defined. This is the intended use case, but I would like to add a check, whether it's called from the same module (in which case Set-Variable -scope 1 should work), when I have found out how to do so.

• I've renamed the parameter -Mean to -Value, for consistency with Set-Variable.

• The function could be extended to optionally set the Private, ReadOnly and AllScope flags. Simply add the desired values to the 3rd argument of the PSVariable constructor, which is called in the above script through New-Object.

PowerShell v5.0 should allow

[static] [int] $variable = 42

[static] [DateTime] $thisday

and the like.