Function overloading in PowerShell

Question

Can you overload functions in PowerShell?

I want to my function to accept a string, array or some switch.

An example of what I want:

• Backup-UsersData singleUser
• Backup-UsersData @('Alice', 'Bob', 'Joe')
• Backup-UsersData -all

Answer 1

In PowerShell functions are not overloaded. The last definition overrides the previous in the same scope or hides the previous in a parent scope. Thus, you should create a single function and provide a way to distinguish its call mode by arguments.

In V2 you may use an advanced function, see help about_Functions_Advanced_Parameters and avoid some manual coding on resolving parameter set ambiguities:

# advanced function with 3 parameter sets function Backup-UsersData (     [Parameter(Position=0, ParameterSetName="user")]     [string]$user,     [Parameter(Position=0, ParameterSetName="array")]     [object[]]$array,     [Parameter(Position=0, ParameterSetName="all")]     [switch]$all ) {     # use this to get the parameter set name     $PSCmdlet.ParameterSetName }  # test Backup-UsersData -user 'John' Backup-UsersData 1, 2 Backup-UsersData -all  # OUTPUT: # user # array # all 

Note that this mechanism is sometimes strange. For example in the first test we have to specify parameter name -user explicitly. Otherwise:

Backup-UsersData : Parameter set cannot be resolved using the specified named parameters. At C:\TEMP\_101015_110059\try2.ps1:21 char:17 + Backup-UsersData <<<<  'John'     + CategoryInfo          : InvalidArgument: (:) [Backup-UsersData], ParentContainsErrorRecordException     + FullyQualifiedErrorId : AmbiguousParameterSet,Backup-UsersData 

In many cases standard, not advanced, function with mixed parameters will do:

function Backup-UsersData (     [string]$user,     [object[]]$array,     [switch]$all ) {     if ($user) {'user'}     elseif ($array) {'array'}     elseif ($all) {'all'}     else {'may be'} }  Backup-UsersData -user 'John' Backup-UsersData -array 1, 2 Backup-UsersData -all Backup-UsersData 

But in this case you should resolve (or accept and ignore) ambiguities, e.g. to decide what to do if, say:

Backup-UsersData -user 'John' -array 1, 2 -all 

Answer 2

Here is a variant of Roman's answer that I think is a little more flexible:

function Backup {     [CmdletBinding(DefaultParameterSetName='Users')]     Param (         [parameter(mandatory=$true, ParameterSetName='Users', position=0, ValueFromPipeline=$true)][string[]]$User,         [parameter(mandatory=$true, ParameterSetName='AllUsers')][switch]$All     )      Begin     {         if ($All) { $User = @('User1', 'User2', 'User3') }     }      Process     {         foreach ($u in $User)         {             echo "Backup $u"         }     } }