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dsc-community-style-guidelines-powershell.instructions.md

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description applyTo
Guidelines for writing PowerShell scripts and modules.
**/*.ps?(m|d)1

PowerShell Guidelines

Naming

  • Use descriptive names (3+ characters, no abbreviations)
  • Functions: PascalCase with Verb-Noun format using approved verbs
  • Parameters: PascalCase
  • Variables: camelCase
  • Keywords: lower-case
  • Classes: PascalCase
  • Include scope for script/global/environment variables: $script:, $global:, $env:

File naming

  • Class files: ###.ClassName.ps1 format (e.g. 001.SqlReason.ps1, 004.StartupParameters.ps1)

Formatting

Indentation & Spacing

  • Use 4 spaces (no tabs)
  • One space around operators: $a = 1 + 2
  • One space between type and variable: [String] $name
  • One space between keyword and parenthesis: if ($condition)
  • No spaces on empty lines
  • Try to limit lines to 120 characters

Braces

  • Newline before opening brace (except variable assignments)
  • One newline after opening brace
  • Two newlines after closing brace (one if followed by another brace or continuation)

Quotes

  • Use single quotes unless variable expansion is needed: 'text' vs "text $variable"

Arrays

  • Single line: @('one', 'two', 'three')
  • Multi-line: each element on separate line with proper indentation
  • Do not use the unary comma operator (,) in return statements to force an array

Hashtables

  • Empty: @{}
  • Multi-line: each property on separate line with proper indentation
  • Properties: Use PascalCase

Comments

  • Single line: # Comment (capitalized, on own line)
  • Multi-line: <# Comment #> format (opening and closing brackets on own line), and indent text
  • No commented-out code

Comment-based help

  • Always add comment-based help to all functions and scripts
  • Comment-based help: SYNOPSIS, DESCRIPTION (40+ chars), PARAMETER, EXAMPLE sections before function/class
  • Comment-based help indentation: keywords 4 spaces, text 8 spaces
  • Include examples for all parameter sets and combinations
  • INPUTS: List each pipeline‑accepted type (one per line) with a 1‑line description.
  • OUTPUTS: List each return type (one per line) with a 1‑line description. Must match both [OutputType()] and actual returns.
  • .NOTES: Include only if it conveys critical info (constraints, side effects, security, version compatibility, breaking behavior). Keep to ≤2 short sentences.

Functions

  • Avoid aliases (use full command names)
  • Avoid Write-Host (use Write-Verbose, Write-Information, etc.)
  • Avoid Write-Output (use return instead)
  • Avoid ConvertTo-SecureString -AsPlainText in production code
  • Don't redefine reserved parameters (Verbose, Debug, etc.)
  • Include a Force parameter for functions that uses $PSCmdlet.ShouldContinue or $PSCmdlet.ShouldProcess
  • For state-changing functions, use SupportsShouldProcess
    • Place ShouldProcess check immediately before each state-change
    • $PSCmdlet.ShouldProcess must use required pattern
  • Use $PSCmdlet.ThrowTerminatingError() for terminating errors (except for classes), use relevant error category
  • Use Write-Error for non-terminating errors, use relevant error category
  • Use Write-Warning for warnings
  • Use Write-Debug for debugging information
  • Use Write-Verbose for actionable information
  • Use Write-Information for informational messages.
  • Never use backtick as line continuation in production code.

ShouldProcess Required Pattern

  • Ensure $descriptionMessage explains what will happen
  • Ensure $confirmationMessage succinctly asks for confirmation
  • Keep $captionMessage short and descriptive (no trailing .)
$descriptionMessage = $script:localizedData.FunctionName_Action_ShouldProcessDescription -f $param1, $param2
$confirmationMessage = $script:localizedData.FunctionName_Action_ShouldProcessConfirmation -f $param1
$captionMessage = $script:localizedData.FunctionName_Action_ShouldProcessCaption

if ($PSCmdlet.ShouldProcess($descriptionMessage, $confirmationMessage, $captionMessage))
{
    # state changing code
}

Force Parameter Pattern

if ($Force.IsPresent -and -not $Confirm)
{
    $ConfirmPreference = 'None'
}

Structure

<#
    .SYNOPSIS
        Brief description

    .DESCRIPTION
        Detailed description

    .PARAMETER Name
        Parameter description

    .INPUTS
        TypeName

        Description

    .OUTPUTS
        TypeName

        Description
#>
function Get-Something
{
    [CmdletBinding()]
    [OutputType([System.String])]
    param
    (
        [Parameter(Mandatory = $true)]
        [System.String]
        $Name,

        [Parameter()]
        [System.String]
        $OptionalParam
    )

    # Implementation
}

Requirements

  • Include [CmdletBinding()] on every function
  • Parameter block at top
  • Parameter block: param () if empty, else opening/closing parentheses on own lines
  • [OutputType({return type})] for functions with output, no output use [OutputType()]
  • All parameters use [Parameter()] attribute, mandatory parameters use [Parameter(Mandatory = $true)]
  • Parameter attributes on separate lines
  • Parameter type on line above parameter name
  • Parameters separated by blank line
  • Parameters should use full type name.
  • Pipeline parameters (ValueFromPipeline = $true) must be declared in ALL parameter sets

Best Practices

Code Organization

  • Use named parameters in function calls
  • Use splatting for long parameter lists
  • Limit piping to one pipe per line
  • Assign function results to variables rather than inline calls
  • Return a single, consistent object type per function

Security & Safety

  • Use PSCredential for credentials
  • Avoid hardcoded computer names, use cross-platform Get-ComputerName instead of $env:COMPUTERNAME
  • Place $null on left side of comparisons
  • Avoid empty catch blocks (instead use -ErrorAction SilentlyContinue)
  • Don't use Invoke-Expression (use & operator)
  • Use CIM commands instead of WMI commands

Variables

  • Avoid global variables (exception: $global:DSCMachineStatus)
  • Use declared variables more than once
  • Avoid unnecessary type declarations when type is clear
  • Use full type name when type casting
  • No default values for mandatory or switch parameters

File Rules

  • End files with only one blank line
  • Use CR+LF line endings
  • Maximum two consecutive newlines

File Encoding

  • Use UTF-8 encoding (no BOM) for all files

Module Manifest

  • Don't use NestedModules for shared commands without RootModule