Pester Test Execution Fails When $WhatIfPreference is Enabled

[zzt108]

Checklist

• Issue has a meaningful title
• I have searched the existing issues. See all issues
• I have tested using the latest version of Pester. See Installation and update guide.

What is the issue?

Problem Description: When $WhatIfPreference is set to $true, Pester test execution fails with multiple side effects:

1. Generates excessive "What if" messages during test discovery and execution
2. Prevents actual command execution
3. Causes test framework to fail silently or with cryptic error messages

Specific Symptoms:

• Test discovery completes
• Test execution fails
• Multiple "What if" messages appear in output
• Error messages like "Cannot bind argument to parameter 'Root' because it is null"

Root Cause: The $WhatIfPreference setting affects command execution globally, interfering with Pester's internal test mechanisms. Simply setting it to $false within a function's local scope does not consistently resolve the issue.

Expected Behavior

$WhatIfPreference should not cause exception

Steps To Reproduce

I use ipynb. I put the following into a cell:

$WhatIfPreference = $true

Invoke-Pester "Firewall.Tests.ps1" 

Describe your environment

Pester version : 5.6.1 C:\Program Files\WindowsPowerShell\Modules\Pester\5.6.1\Pester.psm1
PowerShell version : 5.1.22621.4391
OS version : Microsoft Windows NT 10.0.22631.0

Possible Solution?

this approach works:

$originalGlobalWhatIf = $global:WhatIfPreference
$originalLocalWhatIf = $WhatIfPreference
$originalPSDefaultWhatIf = $PSDefaultParameterValues['*:WhatIf']

try {
    $global:WhatIfPreference = $false
    $WhatIfPreference = $false
    $PSDefaultParameterValues['*:WhatIf'] = $false
    
    # Test execution
}
finally {
    $global:WhatIfPreference = $originalGlobalWhatIf
    $WhatIfPreference = $originalLocalWhatIf
    $PSDefaultParameterValues['*:WhatIf'] = $originalPSDefaultWhatIf
}

[zzt108]

here is the content of Firewall.Tests.ps1:

param(
    [string[]]$FirewallRules = @()
)

Describe "Firewall Rule Validation" {
    BeforeAll {
        $script:rulesToTest = if ($FirewallRules.Count -gt 0) { $FirewallRules } else {
            @(
                "Rule1Name",
                "Rule2Name",
                "Rule3Name"
            )
        }            
    }
    # If no rules are passed, use default rules

    It "Should have required firewall rules" {
        foreach ($ruleName in $script:rulesToTest) {
            $firewallRule = Get-NetFirewallRule -DisplayName $ruleName -ErrorAction SilentlyContinue
            
            $firewallRule | Should -Not -BeNullOrEmpty -Because "Firewall rule '$ruleName' should exist"
            $firewallRule.Enabled | Should -Be $true -Because "Firewall rule '$ruleName' should be enabled"
        }
    }

    # Optional: More specific rule validation
    It "Should have correct rule configurations" {
        foreach ($ruleName in $script:rulesToTest) {
            $firewallRule = Get-NetFirewallRule -DisplayName $ruleName -ErrorAction SilentlyContinue
            $firewallRule | Should -Not -BeNullOrEmpty -Because "Firewall rule '$ruleName' should exist"            
            if ($firewallRule) {
                # Example additional checks
                $firewallRule.Action | Should -Be "Allow" -Because "Rule '$ruleName' should allow traffic"
                # Add more specific assertions as needed
            }
        }
    }
}

[nohwnd]

Cool that no-one saw that before. We should either detect that and throw, or pass the variable down skipping the pester stuff. Both solutions seem bad.

[zzt108]

I guess somewhere $PSCmdlet.ShouldProcess prevents execution of important initialization steps for registry mocking. If those important steps would be executed even if $WhatIfPreference is true, then the problem would be solved.

Another possibility is to save the state of $WhatIfPreference before execution, setting it to $false and finally restoring, as in the example I provided

[nohwnd]

Sorry I meant bad == hard to implement and track. And relatively expensive to restore the whatIf before every user scope executed script block.

[zzt108]

Then maybe remains throwing an exception with an explanation?