Function overloading in PowerShell - function

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"
}
}
}

Answer 3

1) Build a class...
class c1 {
[int]f1( [string]$x ){ return 1 }
[int]f1( [int ]$x ){ return 2 }
}
1+) Use STATIC METHODS if you prefer to call them without instantiation...
class c1 {
static [int] f1( [string]$x ){ return 1 }
static [int] f1( [int]$x ){ return 2 }
}
2) Call the methods in class or object... overload works OK
$o1 = [c1]::new()
o1.f1( "abc" ) ~> returns 1
o1.f1( 123 ) ~> returns 2
-OR-
[c1]::f1( "abc" ) ~> returns 1
[c1]::f1( 123 ) ~> returns 2
3)
If (like me)
you want to have "Overloaded Functions" placed in a libraries...
so your users can use them transparently...
from code or from Interactive Command Line (REPL)...
the closest I could came to
"Overloading functions in Powershell"
was something like this:
function Alert-String() { [c1]::f1( "abc" ) }
function Alert-Strings(){ [c1]::f1( 123 ) }
function Alert-Stringn(){ [c1]::f1( 123 ) }
Maybe in PS-Core v8??? ;-)
Hope it helps...

Answer 4

If you use PSObject instead of Object to define your parameter type, it should work.
For example, The function Get-Control, know's how to overload based on type string or template and can be called using the positional value:
Get-Control "A-Name-Of-A-Control"
Get-Control $template
To make the overload work, use PSObject as follows:
Function Get-Control {
Param(
[Parameter(Mandatory=$False,ParameterSetName="ByTemplate",Position=0)]
[PSObject]
$Template,
[Parameter(Mandatory=$False,ParameterSetName="ByName",Position=0)]
[String]
$Name,
[Parameter(Mandatory=$False)]
[Switch]
$List
)
... # remaining code removed for brevity