Writing an Advanced Function

An Advanced Function is one that contains either a [cmdletbinding()] attribute or the Parameter attribute, or both.
[cmdletbinding()] adds cmdlet features to the function: common parameters (-verbose, -whatif, -confirm etc)

Attributes

Several (optional) attributes can be added to the Param clause of an advanced function.
The Param keyword is used to define parameters in both simple and advanced functions,
The Parameter attribute in advanced functions is to validate (limit the valid values of each parameter).

[parameter(Argument=value)] # Define attributes for each parameter, see below for a list of Parameter Arguments.
[alias("CN","MachineName")] # Establish an alternate name(s) for the parameter.
[AllowNull()]
[AllowEmptyString()]
[AllowEmptyCollection()]
[ValidateCount(1,5)]
# The number or parameters accepted.
[ValidateLength(1,10)] # minimum and max. number of characters in a parameter or variable value.
[ValidatePattern("[0-9][0-9][0-9][0-9]")] # specifies a regex that is compared to the parameter or variable value.
[ValidateRange(0,10)] # specifies a numeric range for each parameter or variable value.
[ValidateScript({$_ -ge (get-date)})] # specifies a script that is used to validate a parameter or variable value. [ValidateSet("Low", "Average", "High")] # specifies a set of valid values for a parameter or variable.
[ValidateNotNull()] # specifies that the parameter value cannot be null ($null).
[ValidateNotNullOrEmpty()] # specifies that the parameter value cannot be null and cannot be an empty string ("")
DynamicParam {<statement-list>} # A dynamic parameter.
[Switch]<ParameterName> # Set a parameter with only a True/False value.

Parameter arguments

These can be included in each [Parameter( )] attribute. The Parameter attribute is optional, and you can omit it if none of the parameters of your advanced function need attributes. Separate multiple arguments with commas.

Mandatory=$true # Whether the parameter is mandatory or optional (the default) If you call a function without passing a value for a mandatory parameter, PowerShell will prompt for the value.
Position=0
# By default, PowerShell assigns each parameter a position number in the order declared, starting from 0.
ParameterSetName="User"
# allow a function to accept different sets of parameters for different tasks.
ValueFromPipeline=$true
# Accept values via the pipeline.
ValueFromPipelineByPropertyName=$true
# Accept values via the pipeline of the same type expected by the parameter and which also must have the same name as the parameter accepting pipeline input.
ValueFromRemainingArguments=$true
HelpMessage="Custom prompt message that appears for a mandatory parameter."

Below is a template for an Advanced Function, containing help text, parameters and begin-process-end blocks:

Function FnNameVERB-NOUN {
<#
.SYNOPSIS
    This advanced function does blah

.DESCRIPTION
    A more detailed description of what the function does.

.PARAMETER DemoParam1
    The parameter DemoParam1 is used to define the value of blah and also blah.

.PARAMETER DemoParam2
    The parameter DemoParam2 is used to define the value of blah and also blah.

.EXAMPLE
    The example below does blah
    PS C:\> <Example>
    
.NOTES
    Author: <Name>
    Last Edit: yyyy-mm-dd
    Version 1.0 - initial release of blah
    Version 1.1 - update for blah

#>
    [CmdletBinding()]  # Add cmdlet features.
    Param (
        # Define parameters below, each separated by a comma

        [Parameter(Mandatory=$True)]
        [int]$DemoParam1,

        [Parameter(Mandatory=$False)]
        [ValidateSet('Alpha','Beta','Gamma')]
        [string]$DemoParam2,
 
        # you don’t have to use the full [Parameter()] decorator on every parameter
        [string]$DemoParam3,
        
        [string]$DemoParam4, $DemoParam5
        
        # Add additional parameters here.
    )

    Begin {
        # Start of the BEGIN block.
        Write-Verbose -Message "Entering the BEGIN block [$($MyInvocation.MyCommand.CommandType): $($MyInvocation.MyCommand.Name)]."

        # Add additional code here.
        
    } # End Begin block

    Process {
        # Start of PROCESS block.
        Write-Verbose -Message "Entering the PROCESS block [$($MyInvocation.MyCommand.CommandType): $($MyInvocation.MyCommand.Name)]."


        # Add additional code here.

        
    } # End of PROCESS block.

    End {
        # Start of END block.
        Write-Verbose -Message "Entering the END block [$($MyInvocation.MyCommand.CommandType): $($MyInvocation.MyCommand.Name)]."

        # Add additional code here.

    } # End of the END Block.
} # End Function

“If you want to go fast, go alone,
If you want to go far, go together” ~ African proverb

Related PowerShell Commands:

Microsoft docs: About Functions Advanced Parameters
$PSCmdlet variable
The ParameterSetName property allows you to see the parameter set that is being used. Parameter sets allow you to create a function that performs different tasks based on the parameters that are specified when the function is run.
Scriptblock - A collection of statements
Ref vars - Passing a reference variable to a function.
TechNet - Defining Parameters


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