Function overloading in powershell

Question

Can you overload functions in powershell ?

What I want to do is that my functin could accept string, array or some switch.

Example what I want:

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

Best regards.

Answer 1

Edited: disregard the answer below. In Payette's book about PowerShell v2, he states that overloading is not supported in V1, which made me wrongly assume that it was in V2. But it's not.

---

This is possible in Powershell V2.

function Backup-UsersData ([string] $user) ...

function Backup-UsersData ([object[]] $array)...

function Backup-UsersData ([switch] $all) ...

In V1, you can test for the variable type (and the switch) explicitly.

Answer 2

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 3

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"
        }
    }
}