Skeleton for ShouldProcess in PowerShell functions

Test-ShouldProcess.ps1

Function New-Something {}

Function Test-ShouldProcess {
    [CmdletBinding(SupportsShouldProcess, ConfirmImpact='Medium')]

    Param(
        [Parameter()]
        [switch]
        $Force
    )

    Begin {
        if (-not $PSBoundParameters.ContainsKey('Verbose')) {
            $VerbosePreference = $PSCmdlet.SessionState.PSVariable.GetValue('VerbosePreference')
        }
        if (-not $PSBoundParameters.ContainsKey('Confirm')) {
            $ConfirmPreference = $PSCmdlet.SessionState.PSVariable.GetValue('ConfirmPreference')
        }
        if (-not $PSBoundParameters.ContainsKey('WhatIf')) {
            $WhatIfPreference = $PSCmdlet.SessionState.PSVariable.GetValue('WhatIfPreference')
        }
        Write-Verbose ('[{0}] Confirm={1} ConfirmPreference={2} WhatIf={3} WhatIfPreference={4}' -f $MyInvocation.MyCommand, $Confirm, $ConfirmPreference, $WhatIf, $WhatIfPreference)
    }

    Process {
        <# Pre-impact code #>
    
        # -Confirm --> $ConfirmPreference = 'Low'
        # ShouldProcess intercepts WhatIf* --> no need to pass it on
        if ($Force -or $PSCmdlet.ShouldProcess("ShouldProcess?")) {
            Write-Verbose ('[{0}] Reached command' -f $MyInvocation.MyCommand)
            # Variable scope ensures that parent session remains unchanged
            $ConfirmPreference = 'None'
            New-Something
        }
        
        <# Post-impact code #>
    }

    End {
        Write-Verbose ('[{0}] Confirm={1} ConfirmPreference={2} WhatIf={3} WhatIfPreference={4}' -f $MyInvocation.MyCommand, $Confirm, $ConfirmPreference, $WhatIf, $WhatIfPreference)
    }
}

Describe 'ShouldProcess' {
    Mock New-Something {}
    It 'Should process by default' {
        Test-ShouldProcess
        Assert-MockCalled New-Something -Scope It -Exactly -Times 1
    }
    It 'Should not process on explicit request for confirmation (-Confirm)' {
        { Test-ShouldProcess -Confirm }
        Assert-MockCalled New-Something -Scope It -Exactly -Times 0
    }
    It 'Should not process on implicit request for confirmation (ConfirmPreference)' {
        {
            $ConfirmPreference = 'Medium'
            Test-ShouldProcess
        }
        Assert-MockCalled New-Something -Scope It -Exactly -Times 0
    }
    It 'Should not process on explicit request for validation (-WhatIf)' {
        { Test-ShouldProcess -WhatIf }
        Assert-MockCalled New-Something -Scope It -Exactly -Times 0
    }
    It 'Should not process on implicit request for validation (WhatIfPreference)' {
        {
            $WhatIfPreference = $true
            Test-ShouldProcess
        }
        Assert-MockCalled New-Something -Scope It -Exactly -Times 0
    }
    It 'Should process on force' {
        $ConfirmPreference = 'Medium'
        Test-ShouldProcess -Force
        Assert-MockCalled New-Something -Scope It -Exactly -Times 1
    }
}