Nicholas Dille

Automation, DevOps and Containerization

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Writing #Pester based Unit Tests for #PowerShell Remoting

Published on 30 May 2016
Tags #PowerShell #pester

I have been writing a lot of pester based unit tests for PowerShell lately. Unfortunately, I am coming across caveats when testing existing code. I have just been testing code involving PowerShell remoting which creates a new context where mocked cmdlets are not available. I will show you how to write unit tests for remoting code.

I was facing several issues when testing remote sessions:

The following code demonstrates how to test a function which is using Invoke-Command to execute remote commands. By mocking Invoke-Command the code can be executed in the current session. Consequently, all other mocked cmdlets work as expected.

Function Invoke-RemoteScriptblock {
    [CmdletBinding()]
    Param()

    Process {
        Invoke-Command -ComputerName 'server' -ScriptBlock {
            (Get-CimInstance -ClassName Win32_OperatingSystem).CSName
        }
    }
}

Describe 'Test remote script blocks' {
    Mock Invoke-Command {
        & $Scriptblock
    }
    Mock Get-CimInstance {
        [pscustomobject]@{
            CSName     = 'server'
            PSTypeName = 'Microsoft.Management.Infrastructure.CimInstance#root/cimv2/Win32_OperatingSystem'
        }
    } -ParameterFilter {$ClassName -And $ClassName -ieq 'Win32_OperatingSystem'}

    It 'Calls remote scriptblock' {
        Invoke-RemoteScriptblock
        Assert-MockCalled Invoke-Command -Scope It -Exactly -Times 1
    }
    It 'Calls command from remote scriptblock' {
        Invoke-RemoteScriptblock
        Assert-MockCalled Get-CimInstance -Scope It -Exactly -Times 1
    }
}

Unfortunately, things get more complicated when using New-PSSession to create the remote session and then use Invoke-Command against it. In this case, you need to mock both cmdlets to handle the remote session. Although I am publishing the code below, it does not work properly because I am still struggling to fake the PSSession object.

Function Invoke-RemoteScriptblock {
    [CmdletBinding()]
    Param()

    Process {
        $Session = New-PSSession -ComputerName 'server'
        Invoke-Command -Session $Session -ScriptBlock {
            (Get-CimInstance -ClassName Win32_OperatingSystem).CSName
        }
    }
}

Describe 'Test remote script blocks' {
    Mock New-PSSession {
        [pscustomobject]@{
            ComputerName      = $ComputerName[0]
            Availability      = 'Available'
            ComputerType      = 'RemoteMachine'
            Id                = 1
            Name              = 'Session1'
            ConfigurationName = 'Microsoft.PowerShell'
            PSTypeName        = 'System.Management.Automation.Runspaces.PSSession'
        }
    }
    Mock Invoke-Command { & $Scriptblock }
    Mock Get-CimInstance {
        [pscustomobject]@{
            CSName     = 'server'
            PSTypeName = 'Microsoft.Management.Infrastructure.CimInstance#root/cimv2/Win32_OperatingSystem'
        }
    } -ParameterFilter {$ClassName -And $ClassName -ieq 'Win32_OperatingSystem'}

    It 'Creates remote session' {
        Invoke-RemoteScriptblock
        Assert-MockCalled New-PSSession -Scope It -Exactly -Times 1
    }
    It 'Calls remote scriptblock' {
        Invoke-RemoteScriptblock
        Assert-MockCalled Invoke-Command -Scope It -Exactly -Times 1
    }
    It 'Calls command from remote scriptblock' {
        Invoke-RemoteScriptblock
        Assert-MockCalled Get-CimInstance -Scope It -Exactly -Times 1
    }
}

When running the above code you will get the following error:

Describing Test remote script blocks
 [-] Creates remote session 130ms
   Cannot process argument transformation on parameter 'Session'. Cannot convert the "@{ComputerName=server; Availability=Available; ComputerType=RemoteMachine; Id=1; Name=Session1; ConfigurationName=Microsoft.PowerShell; State=System.Management.Automation.PSScriptProperty; IdleTimeout=System.Management.Automation.PSScriptProperty; OutputBufferingMode=System.Management.Automation.PSScriptProperty; DisconnectedOn=System.Management.Automation.PSScriptProperty; ExpiresOn=System.Management.Automation.PSScriptProperty}" value of type "System.Management.Automation.Runspaces.PSSession" to type "System.Management.Automation.Runspaces.PSSession[]".
   at line: 34 in C:\Users\ndille\Desktop\Test-NewPSSession.ps1

Apparently, the fake PSSession object cannot be cast for use in Invoke-Command. If you have a solution for this issue, please let me know!

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