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Wednesday, May 17, 2006


PowerShell basic parameter checking of a function

On my blog as I provide samples, I most of the time keep them as simple as possible,
without errorhandling or parameterchecking etc.I just focus on the target

as in the temperature converter function examples in last post,


I did them as one-liners without any checking to keep it simple.

Jeffrey Snover, was so kind to leave a variation in the Comments, that does give usage info if you do not give a parameter (see example 2 below), mine would just convert 0 (see example 1 below)

this makes the script a bit more userfriendly, but what if a string is given ?

FooFoo32 ?

hmm, we should better catch that also then ...(that is why I let them out most of the time, to keep focussed at the "real" example ;-), but ok lets provide 2 other variations now, to make it up)

one way is to give typeinfo data to the parameter, so that this is checked by the function,
(see example 3 below)

so we also did catch wrong parameters now.
but this does not give the usage information,

so I did another veriation, in the last example, I removed the typeinfo to the parameter and used a Trap Statement, to provide the UsageInformation.(see example 4 below)

in the samples below I will show the different version and what does happen with different kinds of (wrong) input :

Examples :

# Example 1 
# My old Function :

function ConvertTo-Celsius ($Fahrenheit) {($Fahrenheit - 32) / 1.8}

# example

MowPS>ConvertTo-Celsius 77

# if you not specify a parameter $null -> 0 is calculated :


# example 2 : 
# Jeffrey Snover 's variation with some errorhandling

function ConvertTo-Fahrenheit ($Celsius=$(Throw "USAGE: ConvertTo-Fahrenheit -Celsius degrees")) {
  "{0:N1}" -f ($Celsius * 1.8 + 32)

function ConvertTo-Celsius ($Fahrenheit=$(Throw "USAGE: ConvertTo-Celsius -Fahrenheit degrees")) {
  "{0:N1}" -f (($Fahrenheit - 32) / 1.8)

# output

# Usage information as no parameter is given, 

USAGE: ConvertTo-Fahrenheit -Celsius degrees
At line:1 char:48
function ConvertTo-Fahrenheit ($Celsius=$(Throw  <<<< "USAGE: ConvertTo-Fahrenheit -Celsius degrees")) {

# but if a string is given :
MowPS>ConvertTo-Fahrenheit foo

# Example 3 
# we can catch this by adding typeinfo to the parameter :

function ConvertTo-Celsius ([decimal]$Fahrenheit=$(Throw "USAGE: ConvertTo-Celsius -Fahrenheit degrees")) {
  "{0:N1}" -f (($Fahrenheit - 32) / 1.8)

# output

USAGE: ConvertTo-Celsius -Fahrenheit degrees
At line:1 char:57
function ConvertTo-Celsius ([decimal]$Fahrenheit=$(Throw  <<<< "USAGE: ConvertTo-Celsius -Fahrenheit degrees")) {

MowPS>ConvertTo-Celsius foo
ConvertTo-Celsius : Cannot convert value "foo" to type "System.Decimal". Error: "Input string was not in a correct format."
At line:1 char:18
+ ConvertTo-Celsius  <<<< foo

MowPS>ConvertTo-Celsius 77

# Example 4 :
# an other variation giving a custom message using TRAP :

function ConvertTo-Fahrenheit ($Celsius=$(Throw "USAGE: ConvertTo-Fahrenheit -Celsius degrees")) {
  trap{Throw "USAGE: ConvertTo-Fahrenheit -Celsius degrees"}
  "{0:N1}" -f ($Celsius * 1.8 + 32)

# output :

MowPS>ConvertTo-Fahrenheit foo
USAGE: ConvertTo-Fahrenheit -Celsius degrees
At line:2 char:13
+   trap{Throw  <<<< "USAGE: ConvertTo-Fahrenheit -Celsius degrees"}

hope this examples help in adapting other examples to scripts with some errorchecking,
(so lazy me can leave them out in my samples and say it is for readability ;-))

For more information about errorhandling and debugging in powershell see also this 7 part series on the PowerShell team blog about it :
Debugging Monad Scripts, Part 7 (Final): How Traps Work (links to all former parts are in this post)

Greetings /\/\o\/\/

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