# Advanced Function Optional Parameter Problem
**URL:** https://forums.powershell.org/t/advanced-function-optional-parameter-problem/2719
**Category:** PowerShell Help
**Created:** [June 18, 2014, 2:28am UTC](https://forums.powershell.org/t/advanced-function-optional-parameter-problem/2719 "2014-06-18T02:28:10Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@szarka-istvan](https://forums.powershell.org/u/szarka-istvan)
#### Post date: [June 18, 2014, 2:28am UTC](https://forums.powershell.org/t/advanced-function-optional-parameter-problem/2719/1 "2014-06-18T02:28:10Z")
Hello,
I’ve written an advanced function that creates one or more VMs. I have a mandatory parameter and many optional, with some default values.
The problem is with the optional parameters that have no default values.
For example, there is a SwitchName parameter that is optional and has no default value, because I don’t always want a new VM to connect to a switch.
However, if I don’t supply any value to it, I get this error, even if I use [AllowEmptyString()] and [AllowNull()] in the PARAM block:
```
New-VM : Cannot validate argument on parameter 'SwitchName'. The argument is null or empty. Provide an argument that is not null or empty, and then try the command again.
```
I want the VMs to connect to a switch only if I supply the name of a switch, otherwise I don’t want them to connect anywhere. How can I do this?
These are the relevant snippets from my code (sorry for the formatting, it gets messed up whatever I do):
```
#Some of the parameters:
[CmdletBinding()]
param(
[Parameter(Mandatory,
ValueFromPipeline,
ValueFromPipelineByPropertyName)]
[string[]]
$VMName,
[Parameter(ValueFromPipeline,
ValueFromPipelineByPropertyName)]
[AllowEmptyString()]
[AllowNull()]
[string]
$SwitchName,
#And so on
)
PROCESS
{
foreach ($name in $VMName)
{
$params = @{Name = $name
MemoryStartupBytes = $MemoryStartupBytes
Generation = 2
NewVHDPath = "$VHDFolder\$name\$name.vhdx"
NewVHDSizeBytes = $VHDSizeBytes
BootDevice = $BootDevice
ComputerName = $ComputerName
Path = $VMFolder
SwitchName = $SwitchName}
New-VM @params
$params = @{VMName = $name
DynamicMemoryEnabled = $true
MinimumBytes = $MemoryMinimumBytes
ComputerName = $ComputerName}
Set-VMMemory @params
}
}
```
---
### Author:  [@donj](https://forums.powershell.org/u/donj)
#### Post date: [June 18, 2014, 2:32am UTC](https://forums.powershell.org/t/advanced-function-optional-parameter-problem/2719/2 "2014-06-18T02:32:16Z")
You need to put some logic in your script to not provide the SwitchName parameter is $SwitchName is null.
```
if ($SwitchName) { $params.Add('SwitchName',$SwitchName) }
```
Something along those lines.
The [AllowNull()] means YOUR FUNCTION will accept a null value from $SwitchName. That doesn’t mean New-VM will do so.
---
### Author:  [@bjornhouben](https://forums.powershell.org/u/bjornhouben)
#### Post date: [June 18, 2014, 2:33am UTC](https://forums.powershell.org/t/advanced-function-optional-parameter-problem/2719/3 "2014-06-18T02:33:05Z")
How do you call the function?
---
### Author:  [@system](https://forums.powershell.org/u/system)
#### Post date: [June 18, 2014, 2:34am UTC](https://forums.powershell.org/t/advanced-function-optional-parameter-problem/2719/4 "2014-06-18T02:34:18Z")
The problem is probably when you pass a null / empty value to the New-VM cmdlet, based on what I can see so far. You’d need to construct your $params hashtable so that only the defined parameters are in it when you call New-VM. Something like this:
```
$params = @{
Name = $name
MemoryStartupBytes = $MemoryStartupBytes
Generation = 2
NewVHDPath = "$VHDFolder\$name\$name.vhdx"
NewVHDSizeBytes = $VHDSizeBytes
BootDevice = $BootDevice
ComputerName = $ComputerName
Path = $VMFolder
}
if ($SwitchName) { $params['SwitchName'] = $SwitchName }
New-VM @params
```
Also, you’ll want to get away from using ValueFromPipeline in combination with String parameters. Any object can be represented as a string, which will give you some crazy results when you start trying to pipe objects to this function. Based on what I see so far, ValueFromPipelineByPropertyName should be all you need.
---
### Author:  [@szarka-istvan](https://forums.powershell.org/u/szarka-istvan)
#### Post date: [June 18, 2014, 2:45am UTC](https://forums.powershell.org/t/advanced-function-optional-parameter-problem/2719/5 "2014-06-18T02:45:17Z")
Thank you a lot, it works! I haven’t encountered this amazing technique yet.
Thank you for the advice on pipeline imput too!
---
### Author:  [@dotnVo](https://forums.powershell.org/u/dotnVo)
#### Post date: [May 16, 2024, 8:47pm UTC](https://forums.powershell.org/t/advanced-function-optional-parameter-problem/2719/6 "2024-05-16T20:47:55Z")