Exceptions In PowerShell

WhoMovedMyCheese.ps1

#4 I call this one: where the bleep did they put the cheese?

function Get-FileException {
   param($Path)
   # Assume I'm doing something with files and it throws this exception:
   $fnf = [System.IO.FileNotFoundException]::new("File Not Found", $Path)
   $inf =  [System.Management.Automation.ItemNotFoundException]::new( "Item Not Found", $fnf)
   throw $inf
}

# This is the basic template for a function (since we know about Schrodinger's Exception)
function Test-Exception {
   try {
      # Imagine I'm doing a lot of work, but within that work, I call something like 
      Get-FileException "NoSuchFile.txt"
   } catch { throw }
}


# Now you're trying to use my code, and you have nice simple error handling and logging code like ...

function Test-Handler {
    $Error.Clear()
    try {

       Test-Exception
    } catch {
       Write-Host "$_ " -Fore Red -NoNewLine
    }

}

Test-Handler  
# Everything is wonderful.


# Now maybe you have some other code that throws FileNotFoundException, 
# So you add specific handlers:

function Test-Handler {
    $Error.Clear()
    try {

       Test-Exception

    } catch [System.Management.Automation.ItemNotFoundException] {
       Write-Host "ITEM NOT FOUND! " -Fore Red -NoNewLine
    } catch [System.IO.FileNotFoundException] { 
       Write-Host "FILE NOT FOUND! " -Fore Magenta -NoNewLine
    }

}

Test-Handler  

# Wait, File Not Found!? Why are we catching the InnerException?
# Let's try reversing the catch statements:

function Test-Handler {
    $Error.Clear()
    try {

       Test-Exception

    } catch [System.IO.FileNotFoundException] { 
       Write-Host "FILE NOT FOUND! " -Fore Magenta -NoNewLine
    } catch [System.Management.Automation.ItemNotFoundException] {
       Write-Host "ITEM NOT FOUND! " -Fore Red -NoNewLine
    } 

}

Test-Handler

# No difference, it's still File Not Found.  That's really wierd.
# SO you try the old code, and it's still showing ItemNotFoundException.
# So you start poking around:


function Test-Handler {
    $Error.Clear()
    try {

       Test-Exception

    } catch [System.IO.FileNotFoundException] { 
       Write-Host "FILE NOT FOUND! " -Fore Magenta -NoNewLine
       Write-Host "$_ " -Fore Yellow -NoNewLine
    } catch [System.Management.Automation.ItemNotFoundException] {
       Write-Host "ITEM NOT FOUND! " -Fore Red -NoNewLine
       Write-Host "$_ " -Fore Yellow -NoNewLine
    } finally {
       % -in $Error { $_.Exception.PSTypeNames -join ", " } | % { $_ } | Write-Host -Fore Cyan
    }

}

Test-Handler

# OK, What the heck is going on? 
# $Error has the ItemNotFoundException, but I can't catch it unless I stop catching FileNotFoundException?


# Now ... here's where it get's crazy
# Some other developer looks at your code and says hey ...
# That inner function should explicitly rethrow $_ because that's how I learned it
function Test-Exception {
   try {
      Get-FileException "NoSuchFile.txt"
   } catch { throw $_ }
}

Test-Handler

# WHOAH! Our ItemNotFoundException is back!


# What if I used trap?

function Test-Exception {
   trap { break }
   $fnf = [System.IO.FileNotFoundException]::new("File Not Found")
   $inf =  [System.Management.Automation.ItemNotFoundException]::new( "whatever", $fnf)
   throw $inf
}

Test-Handler 

# Oh, heck no. Now we're back to File Not Found?
# What if we try the { throw $_ } trick that fixed the other one?

function Test-Exception {
   trap { throw $_ }
   $fnf = [System.IO.FileNotFoundException]::new("File Not Found")
   $inf =  [System.Management.Automation.ItemNotFoundException]::new( "whatever", $fnf)
   throw $inf
}

Test-Handler 

# Ok, I don't even know what just happened. $_ was an array?
# Maybe we have to be specific?


function Test-Exception {
   trap [System.Management.Automation.ItemNotFoundException] { break }

   $fnf = [System.IO.FileNotFoundException]::new("File Not Found")
   $inf =  [System.Management.Automation.ItemNotFoundException]::new( "whatever", $fnf)
   throw $inf
}

Test-Handler 

# for the love of ....