Writing #Pester based Unit Tests for #PowerShell Remoting
Published on 30 May 2016Tags #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:
-
For one thing, the remote endpoint may not be available causing the test to fail
-
Commands executed in the remote session cannot be mocked
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!