Handling errors from within Param block

Question

I have a situation in which I need to define custom defined error handlers for errors in param block. For instance I need the function to return an exit value of 1 if there are some errors inside the param block and also print out a custom defined error message.

Below is my code:

function test {
  Param(
    [string]$Name,
    [int]$age
  )

  Begin {
    $ErrorVar = 0
    if (! $Name) {
      Write-Host "Name is a mandatory parameter...please provide a value"
      $ErrorVar = 1
    }
    if (! $age) {
      Write-Host "Age is a mandatory parameter...please provide a value"
      $ErrorVar = 1
    }
  }
  Process {
    if ($ErrorVar -eq 0) {
      Write-Host "My name is $name and my age is $age"
    }
  }
  End {
    if ($ErrorVar -eq 1) {
      return 1
    } else {
      return 0
    }
  }
}

The error handler works properly if I don't use any one of the arguments (Name, Age):

PS> $var = test 
Name is a mandatory parameter...please provide a value
Age is a mandatory parameter...please provide a value

PS> $var = test -Name Subhayan
Age is a mandatory parameter...please provide a value

But the moment when I omit the value to the argument I come across a system defined error message and my error handler is not called:

PS> $var = test -Name -age 32
test : Missing an argument for parameter 'Name'. Specify a parameter of type
'System.String' and try again.
At line:1 char:13
+ $var = test -Name -age 32
+             ~~~~~
    + CategoryInfo          : InvalidArgument: (:) [test], ParameterBindingException
    + FullyQualifiedErrorId : MissingArgument,test

How can I ensure that my custom defined error message is called every time there is any error in the param block, even in cases when I use the argument name but failed to supply a value.

Jonas Andersen · Accepted Answer · 2016-10-06 08:28:23Z

1

You should read about Advanced Functions in PowerShell. This can be done here: https://technet.microsoft.com/en-us/library/hh847743.aspx

As for your function, you could simply use the Mandatory keyword inside a Parameter-block. Like this:

function Test 
{
  Param(
    [Parameter(Mandatory)]
    [String]$Name,
    [Parameter(Mandatory)]
    [Int]$Age
    )
  Process
  {
    Write-Host "My name is $name and my age is $age"
  }
}

The documentation also explains if you need a specific range (ValidateRange) or a specific set (ValidateSet).

answered Oct 6, 2016 at 8:28

Jonas Andersen

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2 Comments

Subhayan Bhattacharya Over a year ago

Thanks Jonas . But i need to make the errors from inside Param block custom defined , since i need to redirect the errors to log files. I am failing to understand how can a mandatory keyword help in my case.

Jonas Andersen Over a year ago

The code you posted only tested if the parameters were not null. That check can be done by using the Mandatory-keyword. If you want to check and throw a truly custom message, you should use ValidateScript. In this validation, you input a scriptblock.

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