powershell constraints on arguments

Question

I need a little help on how I can enforce the following constraints on the arguments to a powershell script. Can I specify these constraints with in the param section.

1. At least one argument constraint
2. At most one argument constraint

For example, (just an example, it's not what I am doing) to a script called ReadPlainText.ps1, I would like to give only either of the two arguments: Lines or Chars, but not both. The command ReadPlainText.ps1 Sample.txt -Lines 20 -Chars 10 should result in an error. Similarly, the command ReadPlainText.ps1 Sample.txt should result in error.

Answer 1

You can, in PSH V2, do this will parameter attributes and by putting the two parameters into different parameter sets.

A parameter set is a group of parameters that go together, a command can have multiple parameter sets and only one is available. Parameters not assigned to a parameter group are available across all parameters. This can be seen in the standard cmdlets (removing the common parameters):

PS> gcm -syn get-content

Get-Content [-Path]  …
Get-Content [-LiteralPath]  …

To achive this in a script or function:

• Add [CmdletBinding] as the first non-comment. (A default parameter set can be specified here.)
• In the param block decorate parameters with the Parameter attribute to specify mandatory and the parameter set.

Thus:

[CmdletBinding]
param (
  [parameter(mandatory=$true,ParameterSetName='lines')
  [int]$Lines,
  [parameter(mandatory=$true,ParameterSetName='chars')
  [int]$|Chars
)

To access the parameter set in use $PSCmdlet which gives access to the same information available within cmdlets written C# or VB.

Answer 2

This Example (Source: PowerShell Script – Marking a parameter as required/optional) might help you ...

param(
      [string] $param1 = $(throw "required param"), #throw exception if no value provided.
      [string] $param2, #an optional parameter with no default value.
      [string] $param3 = "default value", #an optional parameter with a default value.
      [string] $param4 = $(Read-Host -prompt "param4 value"), #prompt user for value if none provided.
      [string] $param5 = $( if($thisinput -eq "somevalue") { "defaultvalue1" } else { "defaultvalue2" } ) #conditional default value  
      )