Using null in a string

Question

I've found that using $null is impossible for string in PowerShell. While I've worked with workaround as below I wonder if there's better way?

Essentially I need 3 states of string $null, '' (empty), 'some string string'.

First one means - do not execute.
2nd one - clear text
Replace Text with new value

Below is workaround code where I opted for switch -ClearText. Keep in mind that while I could just not add the Set-WordTextText if field is $null but since I'm doing this in nested scenario I would prefer doing it as $null.

function Remove-WordText {
    Param(
        [parameter(ValueFromPipelineByPropertyName, ValueFromPipeline)][Xceed.Words.NET.InsertBeforeOrAfter] $Paragraph,
        [int] $Index = 0,
        [int] $Count = $($Paragraph.Text.Length),
        [bool] $TrackChanges,
        [bool] $RemoveEmptyParagraph,
        [bool] $Supress = $false
    )
    if ($Paragraph -ne $null) {
        Write-Verbose "Remove-WordText - Current text $($Paragraph.Text) "
        Write-Verbose "Remove-WordText - Removing from $Index to $Count - Paragraph Text Count: $($Paragraph.Text.Length)"
        $Paragraph.RemoveText($Index, $Count, $TrackChanges, $RemoveEmptyParagraph)
    }
    if ($Supress) { return } else { return $Paragraph }
}

function Set-WordTextText {
    param(
        [parameter(ValueFromPipelineByPropertyName, ValueFromPipeline)][Xceed.Words.NET.InsertBeforeOrAfter] $Paragraph,
        [alias ("S")] [AllowNull()][string] $Text,
        [switch] $ClearText,
        [bool] $Supress = $false
    )
    if ($Paragraph -ne $null) {
        if (-not [string]::IsNullOrEmpty($Text)) {
            if ($ClearText -eq $true) {
                Write-Verbose 'Set-WordTextText - Clearing Text $ClearText is True'
                $Paragraph = Remove-WordText -Paragraph $Paragraph
            }
            Write-Verbose "Set-WordTextText - Appending Value $Text"
            $Paragraph = $Paragraph.Append($Text)
        }
    }
    if ($Supress) { return } else { return $Paragraph }
}

I essentially execute it within function:

$Paragraph[$i] = $Paragraph[$i] | Set-WordTextText -Text $Text[$i] -ClearText:$ClearText -Supress $false
$Paragraph[$i] = $Paragraph[$i] | Set-WordTextColor -Color $Color[$i] -Supress $false
$Paragraph[$i] = $Paragraph[$i] | Set-WordTextFontSize -FontSize $FontSize[$i] -Supress $false
$Paragraph[$i] = $Paragraph[$i] | Set-WordTextFontFamily -FontFamily $FontFamily[$i] -Supress $false
$Paragraph[$i] = $Paragraph[$i] | Set-WordTextBold -Bold $Bold[$i] -Supress $false
$Paragraph[$i] = $Paragraph[$i] | Set-WordTextItalic -Italic $Italic[$i] -Supress $false

It only executes action if value is not null. So the "wrapper" function can have from 1 to 20 parameters and execute only if the parameter is given.

Ansgar Wiechers · Accepted Answer · 2018-07-23 12:36:21Z

3

If you specify the type [String] for a parameter that parameter becomes a typed variable and can't be $null anymore. Even if you pass $null to the parameter the value will automatically be converted to an empty string. To allow a parameter to be either $null or a string (empty or not) you'd need something like this:

function Set-WordTextText {
    Param(
        ...
        [Parameter(Mandatory=$false)]
        [AllowNull()]
        $Text = $null,
        ...
    )

    if ($_ -ne $null -and $_ -isnot [String]) {
        throw 'Invalid argument for parameter -Text.'
    }

    ...
}

answered Jul 23, 2018 at 12:36

Ansgar Wiechers

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1 Comment

MadBoy Over a year ago

Makes sense. For some reason I was so focused on having it typed as [string] that I wouldn't accept no type. Thanks

Theo · Accepted Answer · 2018-07-23 08:28:26Z

0

Deciding what to do if $Paragraph is in one of the three states depends on the function you are performing. Something like this might help

if ($null -eq $Paragraph) { 
    Write-Host "Paragraph is Null, do not execute"
    return
}
elseif ([string]::IsNullOrEmpty($Paragraph.Text)) { 
    # this depends on the function you are in.
    # if inside Remove-WordText() you don't have to do anything because it is already empty
    # you can check the function currently executed with $MyInvocation.MyCommand.Name
    if ($MyInvocation.MyCommand.Name -eq 'Remove-WordText') {
        Write-Host "Paragraph.Text is Null or Empty, clear text"
        $Paragraph.RemoveText($Index, $Count, $TrackChanges, $RemoveEmptyParagraph)
    else {
        # however, if you're inside Set-WordTextText() you need to decide if $Text
        # is not empty aswell and if not, you append the Text tp $Paragraph
        if (-not [string]::IsNullOrEmpty($Text)) {
            Write-Verbose "Set-WordTextText - Appending Value $Text"
            $Paragraph = $Paragraph.Append($Text)
        }
    }
}
else {
    Write-Host "Paragraph.Text has value, appending text"
    $Paragraph = $Paragraph.Append($Text)
}

Hope that helps

answered Jul 23, 2018 at 8:28

Theo

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1 Comment

MadBoy Over a year ago

I don't think I follow your logic. $Paragraph being null is an option and works just fine. It's separate value $Text which is a string can't be null and I need it to be null to stop execution. Otherwise I need to provide additional parameter. If I would be able to use $null I could simply skip providing parameter to function and it wouldn't execute. As it is now if I leave parameter not defined and I would skip using -Clear it would essentially wipe value instead of skipping it.

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