About Functions OutputTypeAttribute

Article
09/19/2022

SHORT DESCRIPTION

Describes an attribute that reports the type of object that the function returns.

LONG DESCRIPTION

The OutputType attribute lists the .NET types of objects that the functions returns. You can use its optional ParameterSetName parameter to list different output types for each parameter set.

The OutputType attribute is supported on simple and advanced functions. It is independent of the CmdletBinding attribute.

The OutputType attribute provides the value of the OutputType property of the System.Management.Automation.FunctionInfo object that the Get-Command cmdlet returns.

The OutputType attribute value is only a documentation note. It is not derived from the function code or compared to the actual function output. As such, the value might be inaccurate.

SYNTAX

The OutputType attribute of functions has the following syntax:

[OutputType([<TypeLiteral>], ParameterSetName="<Name>")]
[OutputType("<TypeNameString>", ParameterSetName="<Name>")]

The ParameterSetName parameter is optional.

You can list multiple types in the OutputType attribute.

[OutputType([<Type1>],[<Type2>],[<Type3>])]

You can use the ParameterSetName parameter to indicate that different parameter sets return different types.

[OutputType([<Type1>], ParameterSetName=("<Set1>","<Set2>"))]
[OutputType([<Type2>], ParameterSetName="<Set3>")]

Place the OutputType attribute statements in the attributes list that precedes the Param statement.

The following example shows the placement of the OutputType attribute in a simple function.

function SimpleFunction2
{
  [OutputType([<Type>])]
  Param ($Parameter1)

  <function body>
}

The following example shows the placement of the OutputType attribute in advanced functions.

function AdvancedFunction1
{
  [OutputType([<Type>])]
  Param (
    [parameter(Mandatory=$true)]
    [String[]]
    $Parameter1
  )


  <function body>
}

function AdvancedFunction2
{
  [CmdletBinding(SupportsShouldProcess=<Boolean>)]
  [OutputType([<Type>])]
  Param (
    [parameter(Mandatory=$true)]
    [String[]]
    $Parameter1
  )


  <function body>
}

EXAMPLES

The following function uses the OutputType attribute to indicate that it returns a string value.

function Send-Greeting
{
  [OutputType([String])]
  Param ($Name)

  "Hello, $Name"
}

To see the resulting output type property, use the Get-Command cmdlet.

PS> (Get-Command Send-Greeting).OutputType

Name                                               Type
----                                               ----
System.String                                      System.String

The following advanced function uses the OutputType attribute to indicate that the function returns different types depending on the parameter set used in the function command.

function Get-User
{
  [CmdletBinding(DefaultParameterSetName="ID")]

  [OutputType("System.Int32", ParameterSetName="ID")]
  [OutputType([String], ParameterSetName="Name")]

  Param (
    [parameter(Mandatory=$true, ParameterSetName="ID")]
    [Int[]]
    $UserID,

    [parameter(Mandatory=$true, ParameterSetName="Name")]
    [String[]]
    $UserName
  )


  <function body>
}

The following example demonstrates that the output type property value displays the value of the OutputType attribute, even when it is inaccurate.

The Get-Time function returns a string that contains the short form of the time in any DateTime object. However, the OutputType attribute reports that it returns a System.DateTime object.

function Get-Time
{
  [OutputType([DateTime])]
  Param (
    [parameter(Mandatory=$true)]
    [Datetime]$DateTime
  )

  $DateTime.ToShortTimeString()
}

The Get-Type method confirms that the function returns a string.

PS> (Get-Time -DateTime (Get-Date)).Gettype().FullName
System.String

However, the OutputType property, which gets its value from the OutputType attribute, reports that the function returns a DateTime object.

PS> (Get-Command Get-Time).OutputType

Name                                      Type
----                                      ----
System.DateTime                           System.DateTime

NOTES

The value of the OutputType property of a FunctionInfo object is an array of System.Management.Automation.PSTypeName objects, each of which have Name and Type properties.

To get only the name of each output type, use a command with the following format.

(Get-Command Get-Time).OutputType | ForEach {$_.Name}

The value of the OutputType property can be null. Use a null value when the output is a not a .NET type, such as a WMI object or a formatted view of an object.