Should Throw - Needs more flexibility/power

[ghostsquad]

Asserting on the expected message in the exception is nice:

{ throw 'foo' } | Should Throw 'foo'

But what if I wanted to examine additional properties of the exception, or even the exception type? I'm forced to do this:

$caughtException = $null
try {
    throw (new-object system.notimplementedexception)
} catch {
    $caughtException = $_
}

$caughtException | Should Not Be $null
$caughtException.Exception.GetType() | Should Be ([System.NotImplementedException])
$caughtException.Exception.Message | Should ...
$caughtException.Exception.CustomProperty | Should ...

Proposed Syntax

$action = { throw (new-object system.notimplementedexception) }
$actualException = $action | Should Throw ([System.NotImplementedException]) "My Message" -PassThru
$actualException.CustomProperty | Should ...

[vors]

👍

[dlwyatt]

The way the Should command is currently written is pretty limited. This sort of expansion would be easier if we were to rewrite Should to use dynamic parameter sets instead of its current parse and command lookup approach. In terms of usage, this would just involve adding a hyphen to the operators:

$action | Should Not Throw becomes $action | Should -Not -Throw

Unfortunately, that would break every single Pester script that exists today, but if I were writing the module from scratch, that's probably how I'd do it.

[ghostsquad]

To support backwords compatibility, you can map one style of Should to the other. Maybe writing a warning that the old detected version of using Should has been deprecated.

#New
Function Should {
    [cmdletbinding(DefaultParameterSetName="Old")]
    param (
        [Parameter(ParameterSetName="New")]
        [Type]$Type,

        [Parameter(ParameterSetName="New")]
        [String]$Msg,

        [Parameter(ParameterSetName="New")]
        [Switch]$Throw,

        [Parameter(Position=0, ParameterSetName="Old")]
        [object]$Old1,

        [Parameter(Position=1, ParameterSetName="Old")]
        [object]$Old2,

        [Parameter(Position=2, ParameterSetName="Old")]
        [object]$Old3,

        [Parameter(Position=3, ParameterSetName="Old")]
        [object]$Old4
    )

    Write-Host ("ParameterSet: {0}" -f $PSCmdlet.ParameterSetName)
    Write-Host "Throw: $Throw"
    Write-Host "Type: $Type"
    Write-Host "Msg: $Msg"
    Write-Host "Old1: $Old1"
    Write-Host "Old2: $Old2"
    Write-Host "Old3: $Old3"
    Write-Host "Old4: $Old4"
}

New Syntax Test

$ Should -Throw -Type ([NotImplementedException]) -Msg 'MyMsg'
ParameterSet: New
Throw: True
Type: System.NotImplementedException
Msg: MyMsg
Old1:
Old2:
Old3:
Old4:

While supporting backwords compatibility, named parameters are required to support the new features.

Old Syntax Test

$ Should Throw 'MyMsg'
ParameterSet: Old
Throw: False
Type:
Msg:
Old1: Throw
Old2: MyMsg
Old3:

[dlwyatt]

That could work.

[dfch]

👍 for a more flexible should throw syntax (especially catching specific exceptions)

[dlwyatt]

I'm going to make this my first priority after we get a few other things merged.

[jlucktay]

I'd also like to be able to specify exception types with the Should Throw assertion. 👍

[dfch]

fyi as this issue does not seem to move very much

I created a PoSH module with Pester extensions that address some of the mentioned issues:

In short, with it you can do something like this:

• { 1 / 0 } | Should ThrowException DivideByZeroException
• { Invoke-InexistentCmdlet } | Should ThrowCategory 'NotFound;
• { 1 / 0 } | Should ThrowException '\w+\.DivideByZeroException$'; (with regex parsing)

See:

• Blog Post: Improving Pester and Exception Assertion
• Examples: Wiki
• Code: biz.dfch.PS.Pester.Assertions

[it-praktyk]

@dfch, can you add information about your module to the newly created wiki article Articles and other resources.

@nohwnd, is it addressed in your Assert project? If yes I don't think so that issue still should be opened.

@nohwnd, please add the Assert project to the list too. Thank you.