Allow use of parameter aliases in Mock -ParameterFilter

[richardnagle]

1. General summary of the issue

Powershell parameters can be defined with aliases:

function Get-Something{    
   param(
        [Alias('Path')]
        [string] $PathName)
   #function body
}

Get-Something -Path c:\temp.txt

When mocking with -ParameterFilter you cannot currently use the alias name to filter, but must use the real parameter name.

Mock Get-Something -ParameterFilter {$PathName = "c:\temp.txt"} {return 123} #works
Mock Get-Something -ParameterFilter {$Path = "c:\temp.txt"} {return 456} #fails

2. Describe Your Environment

Pester version : 4.4.0 3.4.0 3.3.5 C:\Program Files\WindowsPowerShell\Modules\Pester\4.4.0\Pester.psd1 C:\Program Files\WindowsPowerShell\Modules\Pester\3.4.0\Pester.psd1 C:\Program Files\WindowsPowerShell\Modules\Pester\3.3.5\Pester.psd1
PowerShell version : 5.1.15063.1292
OS version : Microsoft Windows NT 10.0.15063.0

3. Expected Behavior

Get-Content has a parameter named -Tail which has an alias -Last

(Get-Command get-content).ParameterSets[0] | select -ExpandProperty parameters | select Name,Aliases | where Name -eq Tail

Name Aliases
---- -------
Tail {Last}

Therefore I would expect this test should pass:

It "Uses parameter aliases in Parameter-Filter" {
   Mock Get-Content {return "default-get-content"}
   Mock Get-Content -ParameterFilter {$Last -eq 100} -MockWith {return "aliased-parameter-name"}

   Get-Content -Path "c:\temp.txt" -Last 100 | Should -Be "aliased-parameter-name"
}                                       

4.Current Behavior

When using an alias in -ParameterFilter the variable never has a value.

The output from the above test is

[-] Uses parameter aliases in Parameter-Filter 50ms         
  Expected strings to be the same, but they were different. 
  Expected length: 22                                       
  Actual length:   19                                       
  Strings differ at index 0.                                
  Expected: 'aliased-parameter-name'                        
  But was:  'default-get-content'                           
  -----------^    

[nohwnd]

Mock is generating param block for the parameter filter, so it should also resolve which aliases are there and define them as well to solve this.

[fflaten]

Looks like problem is in Test-ParameterFilter which tests every Mock to verify if the filter matches. The parameter $ScriptBlock, each Mock's parameterfilter, is unmodified and using the alias, while $BoundParameters, the parameters from the Mock call, has had the Last-alias resolved to the correct parameter name. This makes the filter-test fail because you get the following scenario:

& { param($Path, $Tail) $Last -eq 100 } -Path "c:\temp.txt" -Tail 100
#Last will be $null -> test returns $false -> mock isn't used

So while a Mock supports alias when the Mock is called, the filter itself can't use parameter aliases.

I've played with a possible solution to replace all variables in $ScriptBlock that matches a parameter alias in the mocked command with the official parameter name. However, if someone is using a variable in the filter which is also the alias for a parameter, then this change will break the filter. Ex. with

$Last = 101
Mock Get-Content -ParameterFilter {$Tail -eq $Last } -MockWith {return "aliased-parameter-name"}

then Test-ParameterFilter would evaluate the mock using

& { param($Path, $Tail) $Tail -eq $Tail } -Path "c:\temp.txt" -Tail 100

Not sure how big of a problem it would be. You can write bad filters now as well, but people may not know all the parameter aliases for a cmdlet and use it by mistake.

[renehernandez]

@nohwnd I am going to take a look at this, since it is related to the work done in #1007

[nohwnd]

@renehernandez ok. This should not be complicated but there is a lot of potential for breaking changes and conflicts with read-only variables, as seen in the issue you are mentioning.

[renehernandez]

@nohwnd I think it would be possible to avoid the conflicts by rewriting the alias in the ParameterFilter block to the correct parameter name in the mock definition

[nohwnd]

@renehernandez can you give me an example?

[renehernandez]

@nohwnd I may be off here, but I would expect that the AST of ParameterFilter scriptblock could be iterated and whenever we find an alias, then replace it with the real parameter name from the mock definition.

For conflicting parameter names, after the work in #1007, the parameter name in the mock is prefixed with "_" (e.g. PSEdition), thus it shouldn't introduce any conflicts

[nohwnd]

Okay that is how you meant it. That might work. I was asking because earlier I was talking about setting a variable not only for that param name but also for the aliases, and that would not work. I did not know that by rewriting your mean rewriting AST. The only thing is that AST is not avaiable in powershell v2.

[renehernandez]

@nohwnd, I always forgot checking for powershell 2 because I never use it. The closest option there would be to work with System.Management.Automation.PSParser, but unfortunately it is less capable than an AST so will see how it goes.