positional parameter cannot be found that accepts argument

Question

I have a problem with a script. I check for firmware type and after that I format the hard drive based on that. The problem is that I get an error when it runs the command

New-Partition -DiskNumber 0 -GptType '{ebd0a0a2-b9e5-4433-87c0-68b6b72699c7}' $partsize_param -DriveLetter C

It says

a positional parameter cannot be found that accept argument '-UseMaxiumSize'  
+ categoryinfo       : invalidargument: (:) [New-Partition], ParameterBindingException  
+ fullyqualifiederror: PositionalParameterNotFound, New-Partition

The funny thing is that the command runs just fine if I'm using -UseMaximumSize instead of $partsize_param

Can someone point what is the error that I'm making?

function clean_install_hdd () {
    Switch (Get-BiosType) {
        1       {$firmwaremode='Legacy BIOS'}
        2       {$firmwaremode='UEFI Mode'}
        Default {$firmwaremode='Unknown'}
    }
    Get-Disk

    $PartitionSize = Read-Host "Partition size - How many GB or max to use all available space"

    if ("$PartitionSize" -eq "max") {
        $partsize_param = '-UseMaximumSize'
    } else {
        $partsize_param = '-Size ' + $PartitionSize
    }

    if ("$firmwaremode" -eq "Legacy BIOS") {
        Clear-Disk 0 -RemoveData -RemoveOEM  -Confirm:$false; Initialize-Disk 0 -PartitionStyle MBR -Confirm:$false
        New-Partition -DiskNumber 0 -$partsize_param -DriveLetter C -IsActive | Format-Volume -FileSystem NTFS -NewFileSystemLabel Windows -ShortFileNameSupport $False  -Confirm:$false
    }
    if ("$firmwaremode" -eq "UEFI Mode") {
        Clear-Disk 0 -RemoveData -RemoveOEM  -Confirm:$false; Initialize-Disk 0 -PartitionStyle GPT  -Confirm:$false
        $systemPart = New-Partition -DiskNumber 0 -GptType '{c12a7328-f81f-11d2-ba4b-00a0c93ec93b}' -Size 100MB -DriveLetter S
        & format.com "$($systemPart.DriveLetter):" /FS:FAT32 /Q /Y | Out-Null
        New-Partition -DiskNumber 0 -GptType '{e3c9e316-0b5c-4db8-817d-f92df00215ae}' -Size 128MB
        Write-Host $partsize_param
        New-Partition -DiskNumber 0 -GptType '{ebd0a0a2-b9e5-4433-87c0-68b6b72699c7}' $partsize_param -DriveLetter C | Format-Volume -FileSystem NTFS -NewFileSystemLabel Windows -ShortFileNameSupport $False -Confirm:$false
    }
}​

Answer 1

You can't substitute a parameter name with a string literal, like so:

$p = "-ParamName"
Get-Stuff $p

The parser will interpret the string "-ParamName" as an argument to a positional parameter. Since no positional parameters can be found, the error is thrown.

The proper way to handle dynamic parameter arguments like in your case, is to use splatting:

$params = @{}

if ("$PartitionSize" -eq "max") {
    $params['UseMaximumSize'] = $true
} else {
    $params['Size'] = $PartitionSize
}

New-Partition @params

You can put all the named parameter arguments into the hashtable, or you can splat just a partial set of parameters:

if($firmwaremode -eq "Legacy BIOS") {
    New-Partition -IsActive @params
} else {
    New-Partition @params
}

Answer 2

You can't (easily) use a variable as a parameter name. This is a good situation to use variable splatting. This lets you easily build a dynamic set of parameters in a hash table.

function clean_install_hdd () {
    Switch (Get-BiosType) {
        1       {$firmwaremode='Legacy BIOS'}
        2       {$firmwaremode='UEFI Mode'}
        Default {$firmwaremode='Unknown'}
    }
    Get-Disk

    $PartitionSize = Read-Host "Partition size - How many GB or max to use all available space"

    $Params = @{
        DiskNumber = 0
        DriveLetter = "C"
    };

    if ($PartitionSize -eq "max") {
        $Params.Add("UseMaximumSize",$true);
    } else {
        $Params.Add("Size", $PartitionSize);
    }

    if ("$firmwaremode" -eq "Legacy BIOS") {
        Clear-Disk 0 -RemoveData -RemoveOEM  -Confirm:$false; Initialize-Disk 0 -PartitionStyle MBR -Confirm:$false;
        $Params.Add("IsActive",$true);
        New-Partition @Params | Format-Volume -FileSystem NTFS -NewFileSystemLabel Windows -ShortFileNameSupport $False  -Confirm:$false
    }
    if ("$firmwaremode" -eq "UEFI Mode") {
        Clear-Disk 0 -RemoveData -RemoveOEM  -Confirm:$false; Initialize-Disk 0 -PartitionStyle GPT  -Confirm:$false
        $systemPart = New-Partition -DiskNumber 0 -GptType '{c12a7328-f81f-11d2-ba4b-00a0c93ec93b}' -Size 100MB -DriveLetter S
        & format.com "$($systemPart.DriveLetter):" /FS:FAT32 /Q /Y | Out-Null
        New-Partition -DiskNumber 0 -GptType '{e3c9e316-0b5c-4db8-817d-f92df00215ae}' -Size 128MB
        Write-Host $partsize_param
        $Params.Add("GptType","{ebd0a0a2-b9e5-4433-87c0-68b6b72699c7}");
        New-Partition @Params | Format-Volume -FileSystem NTFS -NewFileSystemLabel Windows -ShortFileNameSupport $False -Confirm:$false
    }
}