Enable [OutputType] validation

Example.ps1

Set-OutputTypeValidationMode

function Main
{
   Test-ShouldReturnInt32
   Test-ShouldReturnVoid
}

#region function definitions
function Test-ShouldReturnInt32
{
   [OutputType([int])]
   param ()
   
   #Warning: Should return [int]
   return [long] 1
}

function Test-ShouldReturnVoid
{
   [OutputType([void])]
   param ()

   #Warning: Should not return object
   return 1
}

Main

Set-OutputTypeValidationMode.ps1

function Set-OutputTypeValidationMode
{
    [OutputType([void])]
    [CmdletBinding(DefaultParameterSetName = 'Strict')]
    param (
        #[Parameter(Mandatory, ParameterSetName = 'Off')]
        #[switch] $Off,
        #[Parameter(ParameterSetName = 'Strict')]
        #[switch] $Strict = $false,
        #[switch] $Exclude
    )
    Set-StrictMode -Version Latest
    $ErrorActionPreference = "Stop"
    $ExecutionContext.InvokeCommand.PostCommandLookupAction = {
        param (
            $CommandName,
            $CommandLookupEventArgs
        )

        #Don't need to handle internal command
        if ($CommandLookupEventArgs.CommandOrigin -eq [System.Management.Automation.CommandOrigin]::Internal){
            return
        }

        #Skip prompt command
        if ($CommandName -eq 'prompt'){
            return
        }

        
        #Retrive OutputType informations from command
        [Type[]] $outputTypes = [Linq.Enumerable]::ToArray($CommandLookupEventArgs.Command.OutputType).Type | where { $_ -ne $null }
        [string[]] $outputTypeNames = [Linq.Enumerable]::ToArray($CommandLookupEventArgs.Command.OutputType).Name

        #If [OutputType] attribute is not specified. skip validation
        if ($null -eq $outputTypeNames){
            return
        }

        #TODO:Support function parametersets

        #TODO:Support -Passthru parameter
        #$supportPassThru = $CommandLookupEventArgs.Command.Parameters.ContainsKey('Passthru')

        #TODO: Need to handle ParameterSetName overloads([OutputType([<TypeLiteral>], ParameterSetName="<Name>")])
        [bool] $shouldReturnVoid = ($outputTypeNames[0] -eq [void].FullName) #Explicitly declare funtion with [OutputType([void])]
        [bool] $shouldNotReturnVoid = ($outputTypeNames[0] -ne [void].FullName) #Don't contains [OutputType([void])] declaration

        #$CommandLookupEventArgs.StopSearch = $true
        $CommandLookupEventArgs.CommandScriptBlock = {
            [long] $count = 0
               
            #TODO:Convert to filter instead of foreach-object
            & $CommandName @args | foreach {
                #Write object to output stream
                Write-Output -InputObject $_

                $returnObjectType = $_.GetType()

                ++$count
                $isValid = ($outputTypes | foreach { $_.IsAssignableFrom($returnObjectType)}) -eq $true
                if (!$isValid)
                {
                    Write-Warning ("FunctionName = {0}`r`n`tReturn   = {1}`r`n`tExpected = {2}" -f $CommandName, $returnObjectType.FullName, [String]::Join(', ', $outputTypeNames))
                    #TODO: Need to suppress warning for large pipeline inputs
                }
            }

            if ($shouldReturnVoid -and ($count -gt 0)){
                Write-Warning ("Function should not return object!,CommandName={0}" -f $CommandName)
                return
            }

            if ($shouldNotReturnVoid -and ($count -eq 0)){
                Write-Warning ("Function should return object!,CommandName={0}" -f $CommandName)
                return
            }
            #Output Validation is successfully completed.
        }.GetNewClosure()
    }
}