How works OutputType attribute?

function test
{
  [OutputType([Bool])]
  Param ($a)

  return $a
}

$one = test 1
$two = test 0

$one.GetType() # Int32
$two.GetType() # Int32

I expected $one is true, $two is false.

CodePudding user response：

To build on the helpful comments by Abraham Zinala and Mike Shepard:

As the conceptual about_Functions_OutputTypeAttribute help topic indicates:

• The [OutputType()] attribute does not control a function or script's actual output data type.

  • PowerShell offers no way to declare an enforced "return type" the way languages such as C# do: while it makes sense to constrain and document what a function or script outputs ("returns"), it is technically free to output any number of objects, of any type, simply by writing to the success output stream.

  • By contrast, you can enforce the data type of input provided to a function or script, namely by type-constraining its parameter definitions; for instance, you could ensure that arguments passed to your -a parameter are Booleans to begin with, by replacing param($a) with param([bool] $a), as Abraham notes - see the conceptual about_Functions_Advanced_Parameters help topic.

• Instead, the [OutputType()] attribute only has informational character:

  • It adds metadata to a function definition listing the output type(s), which features such as PowerShell's help system and tab-completion can make use of.

  • It is up to the function / script author to make sure that the types listed via [OutputType()] reflect the actual output data types.

    • Therefore, the information may be incorrect - as it was in your case, accidentally.

Therefore, your function needs to ensure that the desired data type is output as part of the output statement:

function test
{
  [OutputType([bool])]
  Param ($a) # $a is allowed to be of any type (no type constraint present)

  # Output a [bool] explicitly.
  # Note: No strict need for `return` here, 
  #       thanks to PowerShell's implicit output behavior.
  [bool] $a
}