Adding PowerShell functions to an object

Question

I have a PowerShell module with a group of functions.

The function createService creates an instance of a service and returns a variable. Several of my functions use the returned value, but I only want one instance of the service so I cannot call createService in each function.

On the command line, I can do $var = createService($string), then call update($var) and it will work properly, but I don't want to force the user to remember to use $var as a parameter.

Is there a way to put these functions in an object/class so the variable can be stored globally and referenced inside each function instead of through parameters?

Roman Kuzmin · Accepted Answer · 2011-12-14 21:03:35Z

I would propose to start the service by the exposed functions, so that a user does even have to care of starting it.

$module = {
    # The only service instance, $null so far
    $script:service = $null

    # Starts the service once and keeps its the only instance
    function Start-MyService {
        if ($null -eq $script:service) {
            "Starting service"
            $script:service = 'MyService'
        }
    }

    # Ensures the service by Start-MyService and then operates on $script:service
    function Update-MyService1 {
        Start-MyService
        "Updating service 1: $script:service"
    }

    # Ensures the service by Start-MyService and then operates on $script:service
    function Update-MyService2 {
        Start-MyService
        "Updating service 2: $script:service"
    }

    Export-ModuleMember -Function Update-MyService1, Update-MyService2
}

$null = New-Module $module

# Starting service
# Updating service 1: MyService
Update-MyService1

# Updating service 2: MyService
Update-MyService2

Rynant · Accepted Answer · 2011-12-14 21:10:48Z

0

In your module, if you assign the service object to a script scoped variable, all functions in the module can access the variable. Here is an example:

$module = {
    function StartNewService {
        $script:service = 'MyService'
    }
    function UpdateService {
        "Updating service: " + $script:service
    }
    Export-ModuleMember -Function StartNewService, UpdateService
}

$null = New-Module $module

# StartNewService creates the service variable.
StartNewService
# UpdateService accesses the service variable created by StartNewService.
UpdateService

If you declare the variable as $global:service, you can access the variable from outside the module as well.

Edit: To address the comments below, here is a more practical example that shows an appropriate situation for sharing a variable among functions in a module. In this case all of the functions in the module depend on the same instance of the $Locations variable. In this example the variable is created outside of the functions, and is kept private by not including it in the Export-ModuleMember command.

Here is a simplified version of my LocationName.psm1

$Locations = @{}

function Save-LocationName {
    param(
        [parameter(Mandatory=$true)]
        [string]$Name
    )
    $Locations[$Name] = $PWD
}

function Move-LocationName {
    param(
        [parameter(Mandatory=$true)]
        [string]$Name
    )
    if($Locations[$Name]) {
        Set-Location $Locations[$Name]
    }
    else {
        throw ("Location $Name does not exist.")
    }
}

New-Alias -Name svln -Value Save-LocationName
New-Alias -Name mvln -Value Move-LocationName

Export-ModuleMember -Function Save-LocationName, Move-LocationName -Alias svln, mvln

With this module a user can give a name to a directory, and move to that location by using the given name. For example if I am at \\server01\c$\Program Files\Publisher\Application\Logs, I can save the location by entering svln logs1. Now if I change my location, I can return to the logs directory with mvln logs1. In this example it would be impractical to use the locations hashtable for input and output since the functions are always working with the same instance.

edited Dec 14, 2011 at 21:10

answered Dec 13, 2011 at 20:22

Rynant

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5 Comments

Andy Arismendi Over a year ago

I discourage implementation of functions that operate on the reliance of shared hidden data like this. If a function should return a value that another should operate on, they should support pipelining. So one function should be New-Service which could be piped to Start-Service or Set-Service. New-Service should return an object type that can be piped to the other functions.

Rynant Over a year ago

@AndyArismendi Generally I agree, but there are valid cases for sharing variables among functions when using Modules.

Andy Arismendi Over a year ago

Since these functions are being exported to the module customer they should adhere to cmdlet standard behavior. If update-service is called first it will obviously yield an desirable result.

Andy Arismendi Over a year ago

Meant to say "undesirable" result.

Rynant Over a year ago

It might not be an undesirable result; we don't know what the OP's implementation of update-service is. I've updated my answer with a practical example where I think it is appropriate to have a shared variable.

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