Mocking Missing Cmdlet ErrorAction with Pester

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Following on from the previous Mocking Missing Cmdlets with Pester post, I also encountered another interesting problem when attempting to mock cmdlets that were not present on the test system. This one is more of an edge-case, hence its own post. Just like last time, the tests worked when I had the Hyper-V cmdlets installed, but failed when running within an Appveyor VM.

It’s probably not uncommon that you will need to ensure that the code under test should throw an error here-and-there. Here is a pseudo-example that tests that Get-VM writes an error when passed with a non-existent VM name:

Describe 'Mocking ErrorAction Example' {
    Function Get-VM { param ($Name) }
    InModuleScope 'xVMHyper-V' {

        It 'Get-VM Throws' {
            Mock Get-VM -ParameterFilter { $Name -eq 'TestVM' } -MockWith { Write-Error 'Oops' }
            { Get-VM -Name 'TestVM' -ErrorAction Stop } | Should Throw;
        }
    }
}

When this test is run it fails:

Describing Mocking ErrorAction Example
 Write-Error 'Oops'  : Oops
At C:\Program Files\WindowsPowerShell\Modules\Pester\Functions\Mock.ps1:709 char:21
+                     & $___ScriptBlock___ @___BoundParameters___ @___ArgumentList ...
+                     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [Write-Error], WriteErrorException
    + FullyQualifiedErrorId : Microsoft.PowerShell.Commands.WriteErrorException 
 [-] Get-VM Throws 176ms
   Expected: the expression to throw an exception
   at line: 6 in D:\Users\Iain\Desktop\PesterDemo.Tests.ps1
Tests completed in 176ms
Passed: 0 Failed: 1 Skipped: 0 Pending: 0

The problem here is that the stub function is not an advanced function and the –ErrorAction preference switch is ignored! This is easily resolved by adding the [CmdletBinding()] attribute to the stub function definition:

Describe 'Mocking ErrorAction Example' {
    Function Get-VM { [CmdletBinding()] param ($Name) }
    InModuleScope 'xVMHyper-V' {

        It 'Get-VM Throws' {
            Mock Get-VM -ParameterFilter { $Name -eq 'TestVM' } -MockWith { Write-Error 'Oops' }
            { Get-VM -Name 'TestVM' -ErrorAction Stop } | Should Throw;
        }
    }
}

Running the test now results in the expected output:

Describing Mocking ErrorAction Example
 [+] Get-VM Throws 177ms
Tests completed in 177ms
Passed: 1 Failed: 0 Skipped: 0 Pending: 0

This is not a Pester issue and it’s not a Powershell issue either. It’s just the way the normal Powershell functions work. But, just in case someone else runs into it I thought it would be worth quickly documenting!

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