Begin, process, end…

Few weeks ago I’ve discovered very strange bug in the way ‘process’ works when used in wrong context. Long story short: someone wrote function with process block, but without begin block. It’s usually OK, but he tried to put some code between param and process block and that gave him error, that is misleading at best:

function Test-BadProcess {            
param ()            
            
# begin {            
$SomeVariable = 'Initial Value'            
# }            
            
process {            
    "Will barf on you! Get-Process does not take scripts! ARGH!"            
}            
}

So how it looks like when you run it?

Oops? Get-Process? When did I used that command? It appears that when you use process in other context than it was designed to, it will automagically add ‘Get-‘ verb and surprise you. What is even more surprising – it will do the same with begin and end. But first it has to have something to run so…:

function Get-Begin {            
    "And now for something completely different!"            
}            
            
function Get-End {            
    "This is the end... My only friend, the end..."            
}            
            
$null;begin            
$null;end

But that’s not all, folks. Unless you use something that PowerShell would not like (e.g. keywords that do not change into commands when context changes) or known already as a command (alias, application) you can just type Noun, and Get- will be added for you. How I’ve found out? Trace-Command told me…

The only question that remains open: why process/ begin/ end used in such context follow this rule? And few other keywords too (more on that later).

BTW: in case you want to fix it yourself: my suggestion is to define three functions in your profile:

foreach ($key in 'begin process end'.Split()) {             
    $Function = @{             
        Path = "function:\$key"             
        Value = [scriptblock]::Create(            
            "throw 'Keyword ''$key'' used in wrong context!'"            
        )            
        Force = $true            
    }            
    New-Item @Function | Out-Null            
}            

Any of them should give you errors that are more accurate than what you get by default… You may want to fix also other surprising keywords that change into commands in context, where you might not expect it:

'Begin', 'Break', 'Catch', 'Continue',            
'Data', 'Do', 'Dynamicparam', 'Else',            
'Elseif', 'End', 'Exit', 'Filter',            
'Finally', 'For', 'Foreach', 'From',             
'Function', 'If', 'In', 'Param',             
'Process', 'Return', 'Switch', 'Throw',            
'Trap', 'Try', 'Until', 'While' | ForEach-Object {            
    "$_";"`$null;$_";"} $_"            
} | ForEach-Object {            
    [Management.Automation.PSParser]::Tokenize($_, [ref]$null) |            
    Add-Member -MemberType NoteProperty -Name Line -Value $_ -PassThru            
} | Where-Object { $_.Type -in 'Command', 'Keyword'} |             
    Format-Table Line, Type -AutoSize -GroupBy Content

Here you can see all keywords and how they switch their keyword-ness on and off.

I’m only sad I forgot to mention this to PowerShell Team during Deep Dive and ask why is that so. Note to self: next time prepare a list of questions before you go to conference like this one, amount of PowerShell goodness my overwhelm you and in the end – you forget what was that you wanted to ask.

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