Safely converting string to bool in PowerShell

Question

I'm trying to convert an argument of my PowerShell script to a boolean value. This line

[System.Convert]::ToBoolean($a)

works fine as long as I use valid values such as "true" or "false", but when an invalid value, such as "bla" or "" is passed, an error is returned. I need something akin to TryParse, that would just set the value to false if the input value is invalid and return a boolean indicating conversion success or failure. For the record, I tried [boolean]::TryParse and [bool]::TryParse, PowerShell doesn't seem to recognize it.

Right now I'm having to clumsily handle this by having two extra if statements.

What surprised me that none of the how-to's and blog posts I've found so far deal with invalid values. Am I missing something or are the PowerShell kids simply too cool for input validation?

Answer 1

$a = 'bla'
$a = ($a -eq [bool]::TrueString).tostring()
$a

False

Answer 2

You could use a try / catch block:

$a = "bla"
try {
  $result = [System.Convert]::ToBoolean($a) 
} catch [FormatException] {
  $result = $false
}

Gives:

> $result
False

Answer 3

TryParse should work as long as you use ref and declare the variable first:

$out = $null
if ([bool]::TryParse($a, [ref]$out)) {
    # parsed to a boolean
    Write-Host "Value: $out"
} else {
    Write-Host "Input is not boolean: $a"
}

Answer 4

just looked for this again and found my own answer - but as a comment so adding as an answer with a few corrections / other input values and also a pester test to verify it works as expected:

Function ParseBool{
    [CmdletBinding()]
    param(
        [Parameter(Position=0)]
        [System.String]$inputVal
    )
    switch -regex ($inputVal.Trim())
    {
        "^(1|true|yes|on|enabled)$" { $true }

        default { $false }
    }
}

Describe "ParseBool Testing" {
    $testcases = @(
        @{ TestValue = '1'; Expected = $true },
        @{ TestValue = ' true'; Expected = $true },
        @{ TestValue = 'true '; Expected = $true },
        @{ TestValue = 'true'; Expected = $true },
        @{ TestValue = 'True'; Expected = $true },
        @{ TestValue = 'yes'; Expected = $true },
        @{ TestValue = 'Yes'; Expected = $true },
        @{ TestValue = 'on'; Expected = $true },
        @{ TestValue = 'On'; Expected = $true },
        @{ TestValue = 'enabled'; Expected = $true },
        @{ TestValue = 'Enabled'; Expected = $true },

        @{ TestValue = $null; Expected = $false },
        @{ TestValue = ''; Expected = $false },
        @{ TestValue = '0'; Expected = $false },
        @{ TestValue = ' false'; Expected = $false },
        @{ TestValue = 'false '; Expected = $false },
        @{ TestValue = 'false'; Expected = $false },
        @{ TestValue = 'False'; Expected = $false },
        @{ TestValue = 'no'; Expected = $false },
        @{ TestValue = 'No'; Expected = $false },
        @{ TestValue = 'off'; Expected = $false },
        @{ TestValue = 'Off'; Expected = $false },
        @{ TestValue = 'disabled'; Expected = $false },
        @{ TestValue = 'Disabled'; Expected = $false }
    )


    It 'input <TestValue> parses as <Expected>' -TestCases $testCases {
        param ($TestValue, $Expected)
        ParseBool $TestValue | Should Be $Expected
    }
}

Answer 5

Another possibility is to use the switch statemement and only evaluate True, 1 and default:

$a = "Bla"
$ret = switch ($a) { {$_ -eq 1 -or $_ -eq  "True"}{$True} default{$false}}

In this if the string equals to True $true is returned. In all other cases $false is returned.

And another way to do it is this:

@{$true="True";$false="False"}[$a -eq "True" -or $a -eq 1]

Source Ternary operator in PowerShell by Jon Friesen