PowerShell Cmdlet Development Guidelines

# PowerShell Cmdlet Development Guidelines

This guide provides PowerShell-specific instructions to help GitHub Copilot generate idiomatic,
safe, and maintainable scripts. It aligns with Microsoft’s PowerShell cmdlet development guidelines.

## Naming Conventions

- **Verb-Noun Format:**
  - Use approved PowerShell verbs (Get-Verb)
  - Use singular nouns
  - PascalCase for both verb and noun
  - Avoid special characters and spaces

- **Parameter Names:**
  - Use PascalCase
  - Choose clear, descriptive names
  - Use singular form unless always multiple
  - Follow PowerShell standard names

- **Variable Names:**
  - Use PascalCase for public variables
  - Use camelCase for private variables
  - Avoid abbreviations
  - Use meaningful names

- **Alias Avoidance:**
  - Use full cmdlet names
  - Avoid using aliases in scripts (e.g., use `Get-ChildItem` instead of `gci`)
  - Document any custom aliases
  - Use full parameter names

### Example - Naming Conventions

```powershell
function Get-UserProfile {
    [CmdletBinding()]
    param(
        [Parameter(Mandatory)]
        [string]$Username,

        [Parameter()]
        [ValidateSet('Basic', 'Detailed')]
        [string]$ProfileType = 'Basic'
    )

    process {
        $outputString = "Searching for: '$($Username)'"
        Write-Verbose -Message $outputString
        Write-Verbose -Message "Profile type: $ProfileType"
        # Logic here
    }
}
```

## Parameter Design

- **Standard Parameters:**
  - Use common parameter names (`Path`, `Name`, `Force`)
  - Follow built-in cmdlet conventions
  - Use aliases for specialized terms
  - Document parameter purpose

- **Parameter Names:**
  - Use singular form unless always multiple
  - Choose clear, descriptive names
  - Follow PowerShell conventions
  - Use PascalCase formatting

- **Type Selection:**
  - Use common .NET types
  - Implement proper validation
  - Consider ValidateSet for limited options
  - Enable tab completion where possible

- **Switch Parameters:**
  - **ALWAYS** use `[switch]` for boolean flags, never `[bool]`
  - **NEVER** use `[bool]$Parameter` or assign default values
  - Switch parameters default to `$false` when omitted
  - Use clear, action-oriented names
  - Test presence with `.IsPresent`
  - Using `$true`/`$false` in parameter attributes (e.g., `Mandatory = $true`) is acceptable

### Example - Parameter Design

```powershell
function Set-ResourceConfiguration {
    [CmdletBinding()]
    param(
        [Parameter(Mandatory)]
        [string]$Name,

        [Parameter()]
        [ValidateSet('Dev', 'Test', 'Prod')]
        [string]$Environment = 'Dev',

        # ✔️ CORRECT: Use `[switch]` with no default value
        [Parameter()]
        [switch]$Force,

         # ❌ WRONG: Shows incorrect default assignment, however this is correct syntax (requires `[switch]` cast).
        [Parameter()]
        [switch]$Quiet = [switch]$true,

        [Parameter()]
        [ValidateNotNullOrEmpty()]
        [string[]]$Tags
    )

    process {
        # Use .IsPresent to check switch state
        if ($Quiet.IsPresent) {
            Write-Verbose "Quiet mode enabled"
        }
    }
}
```

## Pipeline and Output

- **Pipeline Input:**
  - Use `ValueFromPipeline` for direct object input
  - Use `ValueFromPipelineByPropertyName` for property mapping
  - Implement Begin/Process/End blocks for pipeline handling
  - Document pipeline input requirements

- **Output Objects:**
  - Return rich objects, not formatted text
  - Use PSCustomObject for structured data
  - Avoid Write-Host for data output
  - Enable downstream cmdlet processing

- **Pipeline Streaming:**
  - Output one object at a time
  - Use process block for streaming
  - Avoid collecting large arrays
  - Enable immediate processing

- **PassThru Pattern:**
  - Default to no output for action cmdlets
  - Implement `-PassThru` switch for object return
  - Return modified/created object with `-PassThru`
  - Use verbose/warning for status updates

### Example - Pipeline and Output

```powershell
function Update-ResourceStatus {
    [CmdletBinding()]
    param(
        [Parameter(Mandatory, ValueFromPipeline, ValueFromPipelineByPropertyName)]
        [string]$Name,

        [Parameter(Mandatory)]
        [ValidateSet('Active', 'Inactive', 'Maintenance')]
        [string]$Status,

        [Parameter()]
        [switch]$PassThru
    )

    begin {
        Write-Verbose 'Starting resource status update process'
        $timestamp = Get-Date
    }

    process {
        # Process each resource individually
        Write-Verbose "Processing resource: $Name"

        $resource = [PSCustomObject]@{
            Name        = $Name
            Status      = $Status
            LastUpdated = $timestamp
            UpdatedBy   = "$($env:USERNAME)"
        }

        # Only output if PassThru is specified
        if ($PassThru.IsPresent) {
            Write-Output $resource
        }
    }

    end {
        Write-Verbose 'Resource status update process completed'
    }
}
```

## Error Handling and Safety

- **ShouldProcess Implementation:**
  - Use `[CmdletBinding(SupportsShouldProcess = $true)]`
  - Set appropriate `ConfirmImpact` level
  - Call `$PSCmdlet.ShouldProcess()` as close the the changes action
  - Use `$PSCmdlet.ShouldContinue()` for additional confirmations

- **Message Streams:**
  - `Write-Verbose` for operational details with `-Verbose`
  - `Write-Warning` for warning conditions
  - `Write-Error` for non-terminating errors
  - `throw` for terminating errors
  - Avoid `Write-Host` except for user interface text

- **Error Handling Pattern:**
  - Use try/catch blocks for error management
  - Set appropriate ErrorAction preferences
  - Return meaningful error messages
  - Use ErrorVariable when needed
  - Include proper terminating vs non-terminating error handling
  - In advanced functions with `[CmdletBinding()]`, prefer `$PSCmdlet.WriteError()` over `Write-Error`
  - In advanced functions with `[CmdletBinding()]`, prefer `$PSCmdlet.ThrowTerminatingError()` over `throw`
  - Construct proper ErrorRecord objects with category, target, and exception details

- **Non-Interactive Design:**
  - Accept input via parameters
  - Avoid `Read-Host` in scripts
  - Support automation scenarios
  - Document all required inputs

### Example - Error Handling and Safety

```powershell
function Remove-CacheFiles {
    [CmdletBinding(SupportsShouldProcess, ConfirmImpact = 'High')]
    param(
        [Parameter(Mandatory)]
        [string]$Path
    )

    try {
        $files = Get-ChildItem -Path $Path -Filter "*.cache" -ErrorAction Stop
        
        # Demonstrates WhatIf support
        if ($PSCmdlet.ShouldProcess($Path, 'Remove cache files')) {
            $files | Remove-Item -Force -ErrorAction Stop
            Write-Verbose "Removed $($files.Count) cache files from $Path"
        }
    } catch {
        $errorRecord = [System.Management.Automation.ErrorRecord]::new(
            $_.Exception,
            'RemovalFailed',
            [System.Management.Automation.ErrorCategory]::NotSpecified,
            $Path
        )
        $PSCmdlet.WriteError($errorRecord)
    }
}
```

## Documentation and Style

- **Comment-Based Help:** Include comment-based help for any public-facing function or cmdlet. Inside the function, add a `<# ... #>` help comment with at least:
  - `.SYNOPSIS` Brief description
  - `.DESCRIPTION` Detailed explanation
  - `.EXAMPLE` sections with practical usage
  - `.PARAMETER` descriptions
  - `.OUTPUTS` Type of output returned
  - `.NOTES` Additional information

- **Consistent Formatting:**
  - Follow consistent PowerShell style
  - Use proper indentation (4 spaces recommended)
  - Opening braces on same line as statement
  - Closing braces on new line
  - Use line breaks after pipeline operators
  - PascalCase for function and parameter names
  - Avoid unnecessary whitespace

- **Pipeline Support:**
  - Implement Begin/Process/End blocks for pipeline functions
  - Use ValueFromPipeline where appropriate
  - Support pipeline input by property name
  - Return proper objects, not formatted text

- **Avoid Aliases:** Use full cmdlet names and parameters
  - Avoid using aliases in scripts (e.g., use Get-ChildItem instead of gci); aliases are acceptable for interactive shell use.
  - Use `Where-Object` instead of `?` or `where`
  - Use `ForEach-Object` instead of `%`
  - Use `Get-ChildItem` instead of `ls` or `dir`

---

## Full Example: End-to-End Cmdlet Pattern

```powershell
function Remove-UserAccount {
    [CmdletBinding(SupportsShouldProcess = $true, ConfirmImpact = 'High')]
    param(
        [Parameter(Mandatory, ValueFromPipeline)]
        [ValidateNotNullOrEmpty()]
        [string]$Username,

        [Parameter()]
        [switch]$Force
    )

    begin {
        Write-Verbose 'Starting user account removal process'
        $currentErrorActionValue = $ErrorActionPreference
        $ErrorActionPreference = 'Stop'
    }

    process {
        try {
            # Validation
            if (-not (Test-UserExists -Username $Username)) {
                $errorRecord = [System.Management.Automation.ErrorRecord]::new(
                    [System.Exception]::new("User account '$Username' not found"),
                    'UserNotFound',
                    [System.Management.Automation.ErrorCategory]::ObjectNotFound,
                    $Username
                )
                $PSCmdlet.WriteError($errorRecord)
                return
            }

            # ShouldProcess enables -WhatIf and -Confirm support
            if ($PSCmdlet.ShouldProcess($Username, "Remove user account")) {
                # ShouldContinue provides an additional confirmation prompt for high-impact operations
                # This prompt is bypassed when -Force is specified
                if ($Force -or $PSCmdlet.ShouldContinue("Are you sure you want to remove '$Username'?", "Confirm Removal")) {
                    Write-Verbose "Removing user account: $Username"
                    
                    # Main operation
                    Remove-ADUser -Identity $Username -ErrorAction Stop
                    Write-Warning "User account '$Username' has been removed"
                }
            }
        } catch [Microsoft.ActiveDirectory.Management.ADException] {
            $errorRecord = [System.Management.Automation.ErrorRecord]::new(
                $_.Exception,
                'ActiveDirectoryError',
                [System.Management.Automation.ErrorCategory]::NotSpecified,
                $Username
            )
            $PSCmdlet.ThrowTerminatingError($errorRecord)
        } catch {
            $errorRecord = [System.Management.Automation.ErrorRecord]::new(
                $_.Exception,
                'UnexpectedError',
                [System.Management.Automation.ErrorCategory]::NotSpecified,
                $Username
            )
            $PSCmdlet.ThrowTerminatingError($errorRecord)
        }
    }

    end {
        Write-Verbose 'User account removal process completed'
        # Set ErrorActionPreference back to the value it had
        $ErrorActionPreference = $currentErrorActionValue
    }
}
```