About Functions Advanced Parameters

Article
09/19/2022

Short description

Explains how to add parameters to advanced functions.

Long description

You can add parameters to the advanced functions that you write, and use parameter attributes and arguments to limit the parameter values that function users submit with the parameter.

The parameters that you add to your function are available to users in addition to the common parameters that PowerShell adds automatically to all cmdlets and advanced functions. For more information about the PowerShell common parameters, see about_CommonParameters.

Beginning in PowerShell 3.0, you can use splatting with @Args to represent the parameters in a command. Splatting is valid on simple and advanced functions. For more information, see about_Functions and about_Splatting.

Static parameters

Static parameters are parameters that are always available in the function. Most parameters in PowerShell cmdlets and scripts are static parameters.

The following example shows the declaration of a ComputerName parameter that has the following characteristics:

It's mandatory (required).
It takes input from the pipeline.
It takes an array of strings as input.

Param(
    [Parameter(Mandatory=$true,
    ValueFromPipeline=$true)]
    [String[]]
    $ComputerName
)

Attributes of parameters

This section describes the attributes that you can add to function parameters.

All attributes are optional. However, if you omit the CmdletBinding attribute, then to be recognized as an advanced function, the function must include the Parameter attribute.

You can add one or multiple attributes in each parameter declaration. There's no limit to the number of attributes that you can add to a parameter declaration.

Parameter attribute

The Parameter attribute is used to declare the attributes of function parameters.

The Parameter attribute is optional, and you can omit it if none of the parameters of your functions need attributes. But, to be recognized as an advanced function, rather than a simple function, a function must have either the CmdletBinding attribute or the Parameter attribute, or both.

The Parameter attribute has arguments that define the characteristics of the parameter, such as whether the parameter is mandatory or optional.

Use the following syntax to declare the Parameter attribute, an argument, and an argument value. The parentheses that enclose the argument and its value must follow Parameter with no intervening space.

Param(
    [Parameter(Argument=value)]
    $ParameterName
)

Use commas to separate arguments within the parentheses. Use the following syntax to declare two arguments of the Parameter attribute.

Param(
    [Parameter(Argument1=value1,
    Argument2=value2)]
)

If you use the Parameter attribute without arguments, as an alternative to using the CmdletBinding attribute, the parentheses that follow the attribute name are still required.

Param(
    [Parameter()]
    $ParameterName
)

Mandatory argument

The Mandatory argument indicates that the parameter is required. If this argument isn't specified, the parameter is optional.

The following example declares the ComputerName parameter. It uses the Mandatory argument to make the parameter mandatory.

Param(
    [Parameter(Mandatory=$true)]
    [String[]]
    $ComputerName
)

Position argument

The Position argument determines whether the parameter name is required when the parameter is used in a command. When a parameter declaration includes the Position argument, the parameter name can be omitted and PowerShell identifies the unnamed parameter value by its position, or order, in the list of unnamed parameter values in the command.

If the Position argument isn't specified, the parameter name, or a parameter name alias or abbreviation, must precede the parameter value whenever the parameter is used in a command.

By default, all function parameters are positional. PowerShell assigns position numbers to parameters in the order in which the parameters are declared in the function. To disable this feature, set the value of the PositionalBinding argument of the CmdletBinding attribute to $False. The Position argument takes precedence over the value of the PositionalBinding argument for the parameters on which it's declared. For more information, see PositionalBinding in about_Functions_CmdletBindingAttribute.

The value of the Position argument is specified as an integer. A position value of 0 represents the first position in the command, a position value of 1 represents the second position in the command, and so on.

If a function has no positional parameters, PowerShell assigns positions to each parameter based on the order in which the parameters are declared. However, as a best practice, don't rely on this assignment. When you want parameters to be positional, use the Position argument.

The following example declares the ComputerName parameter. It uses the Position argument with a value of 0. As a result, when -ComputerName is omitted from command, its value must be the first or only unnamed parameter value in the command.

Param(
    [Parameter(Position=0)]
    [String[]]
    $ComputerName
)

Note

When the Get-Help cmdlet displays the corresponding Position parameter attribute, the position value is incremented by one.

For example, a parameter with a Position argument value of 0 has a parameter attribute of Position=1.

ParameterSetName argument

The ParameterSetName argument specifies the parameter set to which a parameter belongs. If no parameter set is specified, the parameter belongs to all the parameter sets defined by the function. Therefore, to be unique, each parameter set must have at least one parameter that isn't a member of any other parameter set.

Note

For a cmdlet or function, there is a limit of 32 parameter sets.

The following example declares a ComputerName parameter in the Computer parameter set, a UserName parameter in the User parameter set, and a Summary parameter in both parameter sets.

Param(
    [Parameter(Mandatory=$true,
    ParameterSetName="Computer")]
    [String[]]
    $ComputerName,

    [Parameter(Mandatory=$true,
    ParameterSetName="User")]
    [String[]]
    $UserName,

    [Parameter(Mandatory=$false)]
    [Switch]
    $Summary
)

You can specify only one ParameterSetName value in each argument and only one ParameterSetName argument in each Parameter attribute. To indicate that a parameter appears in more than one parameter set, add additional Parameter attributes.

The following example explicitly adds the Summary parameter to the Computer and User parameter sets. The Summary parameter is optional in the Computer parameter set and mandatory in the User parameter set.

Param(
    [Parameter(Mandatory=$true,
    ParameterSetName="Computer")]
    [String[]]
    $ComputerName,

    [Parameter(Mandatory=$true,
    ParameterSetName="User")]
    [String[]]
    $UserName,

    [Parameter(Mandatory=$false, ParameterSetName="Computer")]
    [Parameter(Mandatory=$true, ParameterSetName="User")]
    [Switch]
    $Summary
)

For more information about parameter sets, see Cmdlet Parameter Sets.

ValueFromPipeline argument

The ValueFromPipeline argument indicates that the parameter accepts input from a pipeline object. Specify this argument if the function accepts the entire object, not just a property of the object.

The following example declares a ComputerName parameter that's mandatory and accepts an object that's passed to the function from the pipeline.

Param(
    [Parameter(Mandatory=$true,
    ValueFromPipeline=$true)]
    [String[]]
    $ComputerName
)

ValueFromPipelineByPropertyName argument

The ValueFromPipelineByPropertyName argument indicates that the parameter accepts input from a property of a pipeline object. The object property must have the same name or alias as the parameter.

For example, if the function has a ComputerName parameter, and the piped object has a ComputerName property, the value of the ComputerName property is assigned to the function's ComputerName parameter.

The following example declares a ComputerName parameter that's mandatory and accepts input from the object's ComputerName property that's passed to the function through the pipeline.

Param(
    [Parameter(Mandatory=$true,
    ValueFromPipelineByPropertyName=$true)]
    [String[]]
    $ComputerName
)

Note

A typed parameter that accepts pipeline input (by Value) or (by PropertyName) enables use of delay-bind script blocks on the parameter.

The delay-bind script block is run automatically during ParameterBinding. The result is bound to the parameter. Delay binding does not work for parameters defined as type ScriptBlock or System.Object, the script block is passed through without being invoked.

You can read about delay-bind script blocks here about_Script_Blocks.md.

ValueFromRemainingArguments argument

The ValueFromRemainingArguments argument indicates that the parameter accepts all the parameter's values in the command that aren't assigned to other parameters of the function.

There's a known issue for using collections with ValueFromRemainingArguments where the passed-in collection is treated as a single element.

The following example demonstrates this known issue. The Remaining parameter should contain one at index 0 and two at index 1. Instead, both elements are combined into a single entity.

function Test-Remainder
{
     param(
         [string]
         [Parameter(Mandatory = $true, Position=0)]
         $Value,
         [string[]]
         [Parameter(Position=1, ValueFromRemainingArguments)]
         $Remaining)
     "Found $($Remaining.Count) elements"
     for ($i = 0; $i -lt $Remaining.Count; $i++)
     {
        "${i}: $($Remaining[$i])"
     }
}
Test-Remainder first one,two

Found 1 elements
0: one two

Note

This issue is resolved in PowerShell 6.2.

HelpMessage argument

The HelpMessage argument specifies a string that contains a brief description of the parameter or its value. PowerShell displays this message in the prompt that appears when a mandatory parameter value is missing from a command. This argument has no effect on optional parameters.

The following example declares a mandatory ComputerName parameter and a help message that explains the expected parameter value.

Param(
    [Parameter(Mandatory=$true,
    HelpMessage="Enter one or more computer names separated by commas.")]
    [String[]]
    $ComputerName
)

Alias attribute

The Alias attribute establishes an alternate name for the parameter. There's no limit to the number of aliases that you can assign to a parameter.

The following example shows a parameter declaration that adds the CN and MachineName aliases to the mandatory ComputerName parameter.

Param(
    [Parameter(Mandatory=$true)]
    [Alias("CN","MachineName")]
    [String[]]
    $ComputerName
)

Parameter and variable validation attributes

Validation attributes direct PowerShell to test the parameter values that users submit when they call the advanced function. If the parameter values fail the test, an error is generated and the function isn't called. You can also use some of the validation attributes to restrict the values that users can specify for variables.

AllowNull validation attribute

The AllowNull attribute allows the value of a mandatory parameter to be $null. The following example declares a ComputerName parameter that can have a null value.

Param(
    [Parameter(Mandatory=$true)]
    [AllowNull()]
    [String]
    $ComputerName
)

AllowEmptyString validation attribute

The AllowEmptyString attribute allows the value of a mandatory parameter to be an empty string (""). The following example declares a ComputerName parameter that can have an empty string value.

Param(
    [Parameter(Mandatory=$true)]
    [AllowEmptyString()]
    [String]
    $ComputerName
)

AllowEmptyCollection validation attribute

The AllowEmptyCollection attribute allows the value of a mandatory parameter to be an empty collection @(). The following example declares a ComputerName parameter that can have an empty collection value.

Param(
    [Parameter(Mandatory=$true)]
    [AllowEmptyCollection()]
    [String[]]
    $ComputerName
)

ValidateCount validation attribute

The ValidateCount attribute specifies the minimum and maximum number of parameter values that a parameter accepts. PowerShell generates an error if the number of parameter values in the command that calls the function is outside that range.

The following parameter declaration creates a ComputerName parameter that takes one to five parameter values.

Param(
    [Parameter(Mandatory=$true)]
    [ValidateCount(1,5)]
    [String[]]
    $ComputerName
)

ValidateLength validation attribute

The ValidateLength attribute specifies the minimum and maximum number of characters in a parameter or variable value. PowerShell generates an error if the length of a value specified for a parameter or a variable is outside of the range.

In the following example, each computer name must have one to ten characters.

Param(
    [Parameter(Mandatory=$true)]
    [ValidateLength(1,10)]
    [String[]]
    $ComputerName
)

In the following example, the value of the variable $number must be a minimum of one character in length, and a maximum of ten characters.

[Int32][ValidateLength(1,10)]$number = 01

ValidatePattern validation attribute

The ValidatePattern attribute specifies a regular expression that's compared to the parameter or variable value. PowerShell generates an error if the value doesn't match the regular expression pattern.

In the following example, the parameter value must contain a four-digit number, and each digit must be a number zero to nine.

Param(
    [Parameter(Mandatory=$true)]
    [ValidatePattern("[0-9][0-9][0-9][0-9]")]
    [String[]]
    $ComputerName
)

In the following example, the value of the variable $number must be exactly a four-digit number, and each digit must be a number zero to nine.

[Int32][ValidatePattern("^[0-9][0-9][0-9][0-9]$")]$number = 1111

ValidateRange validation attribute

The ValidateRange attribute specifies a numeric range for each parameter or variable value. PowerShell generates an error if any value is outside that range. In the following example, the value of the Attempts parameter must be between zero and ten.

Param(
    [Parameter(Mandatory=$true)]
    [ValidateRange(0,10)]
    [Int]
    $Attempts
)

In the following example, the value of the variable $number must be between zero and ten.

[Int32][ValidateRange(0,10)]$number = 5

ValidateScript validation attribute

The ValidateScript attribute specifies a script that is used to validate a parameter or variable value. PowerShell pipes the value to the script, and generates an error if the script returns $false or if the script throws an exception.

When you use the ValidateScript attribute, the value that's being validated is mapped to the $_ variable. You can use the $_ variable to refer to the value in the script.

In the following example, the value of the EventDate parameter must be greater than or equal to the current date.

Param(
    [Parameter(Mandatory=$true)]
    [ValidateScript({$_ -ge (Get-Date)})]
    [DateTime]
    $EventDate
)

In the following example, the value of the variable $date must be greater than or equal to the current date and time.

[DateTime][ValidateScript({$_ -ge (Get-Date)})]$date = (Get-Date)

ValidateSet attribute

The ValidateSet attribute specifies a set of valid values for a parameter or variable. PowerShell generates an error if a parameter or variable value doesn't match a value in the set. In the following example, the value of the Detail parameter can only be Low, Average, or High.

Param(
    [Parameter(Mandatory=$true)]
    [ValidateSet("Low", "Average", "High")]
    [String[]]
    $Detail
)

In the following example, the value of the variable $flavor must be either Chocolate, Strawberry, or Vanilla.

[ValidateSet("Chocolate", "Strawberry", "Vanilla")]
[String]$flavor = "Strawberry"

The validation occurs whenever that variable is assigned even within the script. For example, the following results in an error at runtime:

Param(
    [ValidateSet("hello", "world")]
    [String]$Message
)

$Message = "bye"

ValidateNotNull validation attribute

The ValidateNotNull attribute specifies that the parameter value can't be $null. PowerShell generates an error if the parameter value is $null.

The ValidateNotNull attribute is designed to be used when the type of the parameter value isn't specified or when the specified type accepts a value of $null. If you specify a type that doesn't accept a $null value, such as a string, the $null value is rejected without the ValidateNotNull attribute, because it doesn't match the specified type.

In the following example, the value of the ID parameter can't be $null.

Param(
    [Parameter(Mandatory=$true)]
    [ValidateNotNull()]
    $ID
)

ValidateNotNullOrEmpty validation attribute

The ValidateNotNullOrEmpty attribute specifies that the parameter value can't be $null and can't be an empty string (""). PowerShell generates an error if the parameter is used in a function call, but its value is $null, an empty string (""), or an empty array @().

Param(
    [Parameter(Mandatory=$true)]
    [ValidateNotNullOrEmpty()]
    [String[]]
    $UserName
)

Dynamic parameters

Dynamic parameters are parameters of a cmdlet, function, or script that are available only under certain conditions.

For example, several provider cmdlets have parameters that are available only when the cmdlet is used in the provider drive, or in a particular path of the provider drive. For example, the Encoding parameter is available on the Add-Content, Get-Content, and Set-Content cmdlets only when it's used in a file system drive.

You can also create a parameter that appears only when another parameter is used in the function command or when another parameter has a certain value.

Dynamic parameters can be useful, but use them only when necessary, because they can be difficult for users to discover. To find a dynamic parameter, the user must be in the provider path, use the ArgumentList parameter of the Get-Command cmdlet, or use the Path parameter of Get-Help.

To create a dynamic parameter for a function or script, use the DynamicParam keyword.

The syntax is as follows:

DynamicParam {<statement-list>}

In the statement list, use an If statement to specify the conditions under which the parameter is available in the function.

Use the New-Object cmdlet to create a System.Management.Automation.RuntimeDefinedParameter object to represent the parameter and specify its name.

You can use a New-Object command to create a System.Management.Automation.ParameterAttribute object to represent attributes of the parameter, such as Mandatory, Position, or ValueFromPipeline or its parameter set.

The following example shows a sample function with standard parameters named Name and Path, and an optional dynamic parameter named DP1. The DP1 parameter is in the PSet1 parameter set and has a type of Int32. The DP1 parameter is available in the Get-Sample function only when the value of the Path parameter starts with HKLM:, indicating that it's being used in the HKEY_LOCAL_MACHINE registry drive.

function Get-Sample {
  [CmdletBinding()]
  Param([String]$Name, [String]$Path)

  DynamicParam
  {
    if ($Path.StartsWith("HKLM:"))
    {
      $attributes = New-Object -Type `
        System.Management.Automation.ParameterAttribute
      $attributes.ParameterSetName = "PSet1"
      $attributes.Mandatory = $false
      $attributeCollection = New-Object `
        -Type System.Collections.ObjectModel.Collection[System.Attribute]
      $attributeCollection.Add($attributes)

      $dynParam1 = New-Object -Type `
        System.Management.Automation.RuntimeDefinedParameter("DP1", [Int32],
          $attributeCollection)

      $paramDictionary = New-Object `
        -Type System.Management.Automation.RuntimeDefinedParameterDictionary
      $paramDictionary.Add("DP1", $dynParam1)
      return $paramDictionary
    }
  }
}

For more information, see RuntimeDefinedParameter.

Switch parameters

Switch parameters are parameters with no parameter value. They're effective only when they're used and have only one effect.

For example, the NoProfile parameter of powershell.exe is a switch parameter.

To create a switch parameter in a function, specify the Switch type in the parameter definition.

For example:

Param([Switch]<ParameterName>)

Or, you can use an another method:

Param(
    [Parameter(Mandatory=$false)]
    [Switch]
    $<ParameterName>
)

Switch parameters are easy to use and are preferred over Boolean parameters, which have a more difficult syntax.

For example, to use a switch parameter, the user types the parameter in the command.

-IncludeAll

To use a Boolean parameter, the user types the parameter and a Boolean value.

-IncludeAll:$true

When creating switch parameters, choose the parameter name carefully. Be sure that the parameter name communicates the effect of the parameter to the user. Avoid ambiguous terms, such as Filter or Maximum that might imply a value is required.

ArgumentCompleter attribute

The ArgumentCompleter attribute allows you to add tab completion values to a specific parameter. Like DynamicParameters, the available values are calculated at runtime when the user presses Tab after the parameter name.

To add an ArgumentCompleter attribute, you need to define a script block that will determine the values. The script block must take the following parameters in the order specified below. The parameter's names don't matter as the values are provided positionally.

The syntax is as follows:

Param(
    [Parameter(Mandatory)]
    [ArgumentCompleter({
        param ($commandName, $parameterName, $wordToComplete, $commandAst, $fakeBoundParameter)
        # Perform calculation of tab completed values here.
    })]
)

ArgumentCompleter script block

The script block parameters are set to the following values:

$commandName (Position 0) - This parameter is set to the name of the command for which the script block is providing tab completion.
$parameterName (Position 1) - This parameter is set to the parameter whose value requires tab completion.
$wordToComplete (Position 2) - This parameter is set to value the user has provided before they pressed Tab. Your script block should use this value to determine tab completion values.
$commandAst (Position 3) - This parameter is set to the Abstract Syntax Tree (AST) for the current input line. For more information, see Ast Class.
$fakeBoundParameter (Position 4) - This parameter is set to a hashtable containing the $PSBoundParameters for the cmdlet, before the user pressed Tab. For more information, see about_Automatic_Variables.

The ArgumentCompleter script block must unroll the values using the pipeline, such asForEach-Object, Where-Object, or another suitable method. Returning an array of values causes PowerShell to treat the entire array as one tab completion value.

The following example adds tab completion to the Value parameter. If no $Type is specified, fruits and vegetables are returned. If a $Type is specified, only values for the type are specified. In addition, the -like operator ensures that if the user types the following command and uses Tab completion, only Apple is returned.

Test-ArgumentCompleter -Type Fruits -Value A

function Test-ArgumentCompleter {
[CmdletBinding()]
 param (
        [Parameter(Mandatory)]
        [ValidateSet('Fruits', 'Vegetables')]
        $Type,
        [Parameter(Mandatory)]
        [ArgumentCompleter( {
            param ( $commandName,
                    $parameterName,
                    $wordToComplete,
                    $commandAst,
                    $fakeBoundParameters )

            $possibleValues = @{
                Fruits = @('Apple', 'Orange', 'Banana')
                Vegetables = @('Tomato', 'Squash', 'Corn')
            }
            if ($fakeBoundParameters.ContainsKey('Type'))
            {
                $possibleValues[$fakeBoundParameters.Type] | Where-Object {
                    $_ -like "$wordToComplete*"
                }
            }
            else
            {
                $possibleValues.Values | ForEach-Object {$_}
            }
        } )]
        $Value
      )
}

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About Functions Advanced Parameters

Short description

Long description

Static parameters

Attributes of parameters

Parameter attribute

Mandatory argument

Position argument

ParameterSetName argument

ValueFromPipeline argument

ValueFromPipelineByPropertyName argument

ValueFromRemainingArguments argument

HelpMessage argument

Alias attribute

Parameter and variable validation attributes

AllowNull validation attribute

AllowEmptyString validation attribute

AllowEmptyCollection validation attribute

ValidateCount validation attribute

ValidateLength validation attribute

ValidatePattern validation attribute

ValidateRange validation attribute

ValidateScript validation attribute

ValidateSet attribute

ValidateNotNull validation attribute

ValidateNotNullOrEmpty validation attribute

Dynamic parameters

Switch parameters

ArgumentCompleter attribute

ArgumentCompleter script block

See also

Additional resources