The Parameter attribute is an optional attribute that is used to define the behavior of a parameter within a script or function. Creating an instance of the Parameter object shows the different properties that might be set:

PS> [Parameter]::new()

Position                       : -2147483648
ParameterSetName                : __AllParameterSets
Mandatory                       : False
ValueFromPipeline               : False
ValueFromPipelineByPropertyName : False
ValueFromRemainingArguments     : False
HelpMessage                     :
HelpMessageBaseName             :
HelpMessageResourceId           :
DontShow                        : False
TypeId                          : System.Management.Automation.ParameterAttribute

A few of these properties should be ignored as they are not intended to be set directly, such as HelpMessageBaseName, HelpMessageResourceId, and TypeId.

A number of the more complex properties are explored in other sections in this chapter, such as ParameterSetName, ValueFromPipeline, and ValueFromPipelineByPropertyName.

The Parameter attribute is placed before the parameter itself. The following example shows the simplest use of the Parameter attribute:

[CmdletBinding()]
param (
    [Parameter()]
    $Paramter1
)

Using the Parameter attribute has the side-effect of turning a basic function into an advanced function, even when the CmdletBinding attribute itself is missing. Get-Command may be used to explore whether CmdletBinding is present:

PS> function Test-CmdletBinding {
>>     param (
>>         [Parameter()]
>>         $Parameter1
>>     )
>> }
PS> Get-Command Test-CmdletBinding | Select-Object CmdletBinding

CmdletBinding
-------------
        True

This means that the common parameters, including Verbose and ErrorAction, are available to any function that uses the Parameter attribute (for any parameter).

Starting with PowerShell 3, Boolean properties, such as Mandatory and ValueFromPipeline, may be written without providing an explicit argument. For example, Parameter1 is made mandatory in the following code:

function Test-Mandatory {
    [CmdletBinding()]
    param (
        [Parameter(Mandatory)]
        $Parameter1
    )
}