How to Test Function Arguments

[leewilmott]

I hope someone can help me.

I want to know how to test for invalid parameters supplied to a function. For example, consider the following function:

function Add-Ten {
    param(
        [int]
        $Number
    )
    return $Number + 10
}

The following code details a valid Pester test:

Describe 'Add-Ten' {
    Context "Add-Ten: Testing Valid Numbers" {
        It 'Returns 17' {
            $actual = Add-Ten -Number 7

            $Expected = 17
            $actual | Should -Be $Expected
        }
    }
}

I want to create a Pester test to make sure that any changes to my function doesn't allow strings to be supplied, for example...

Add-Ten -Number 'abcde'

How can I create a Pester test for this? I cannot work out how to do it.

Please help! Many thanks!

[fflaten]

Hi. You may use the Should -Throw assertion for this by examining the error/exception.
See https://pester.dev/docs/assertions/#throw and https://pester.dev/docs/commands/Should#throw

E.g.

Describe 'Add-Ten' {
    Context "Add-Ten: Testing invalid input" {
        It 'Throws when passing non-int value to -Number' {
            # Only check for exception
            { Add-Ten -Number "string" } | Should -Throw

            # Verify specific error message (depends on OS-language)
            { Add-Ten -Number "string" } | Should -Throw -ExpectedMessage '*Cannot convert value "string" to type "System.Int32"*'

            # Verify using error id (found using $error[0].FullyQualifiedErrorId)
            { Add-Ten -Number "string" } | Should -Throw -ErrorId "ParameterArgumentTransformationError,Add-Ten"

            # Overkill mode.  Verify details in the exception :)
            $err = { Add-Ten -Number "string" } | Should -Throw -ErrorId "ParameterArgumentTransformationError,Add-Ten" -PassThru
            # Check that the expected parameter failed
            $err.Exception.ParameterName | Should -Be 'Number'
            $err.Exception.ParameterType | Should -Be ([int])
            # Verify the provided parameter value type
            $err.Exception.TypeSpecified | Should -Be ([string])
        }
    }
}

In this simple example you could just check the parameter type, e.g.
(Get-Command Add-Ten).Parameters['Number'].ParameterType | Should -Be ([int])

[leewilmott]

@fflaten, thank you so much for your response.

I did try -Throw but I couldn't get it to work. In light of your reply I tried as you suggested and it works perfectly:

Describe 'Add-Ten' {
    Context "Add-Ten: Testing invalid input" {
        It 'Throws when passing non-int value to -Number' {
            # Only check for exception
            { Add-Ten -Number "string" } | Should -Throw
        }
    }
}

However, when I was testing (before I posted) I was trying 'something like' this:

Describe 'Add-Ten' {
    Context "Add-Ten: Testing invalid input" {
        It 'Throws when passing non-int value to -Number' {
            # Only check for exception
            { Add-Ten -Number 1.0 } | Should -Throw
        }
    }
}

...but this doesn't pass the test for some reason.

However, I don't think that matters because your suggestion of this is MUCH better:

(Get-Command Add-Ten).Parameters['Number'].ParameterType | Should -Be ([int])

Thank you once again!

[fflaten]

1.0 will automatically be converted to int 1 by PowerShell.

[leewilmott]

Oh of course, just like [int]5.5 returns 6 in PowerShell. I'm feeling a little stupid now! :) Thanks!