Proper method to handle nested loops?

[PleaseStopAsking]

I am running into some issues with my migration from v4 to v5 around usage of -ForEach. One specific scenario is around validation of cmdlet parameters. My top level -ForEach seems to be working as expected but my nested -ForEach neither expands the <_> nor works in any manner and simply throws a CommandNotFoundException. I am clearly missing something here but what that is, is unclear.

BeforeDiscovery {
    $Cmdlets = Get-Command -Module $env:BHProjectName -CommandType 'Cmdlet', 'Function' -ErrorAction 'Stop'
}

Describe 'Help for <_>' -ForEach $Cmdlets {
    BeforeAll {
        $Common = 'Debug', 'ErrorAction', 'ErrorVariable', 'InformationAction', 'InformationVariable', 'OutBuffer', 'OutVariable', 'PipelineVariable', 'Verbose', 'WarningAction', 'WarningVariable', 'Confirm', 'Whatif'
    }
    BeforeEach {
        $Command = Get-Command $_
        $CommandParameters = $Command.ParameterSets.Parameters | Sort-Object -Property Name -Unique | Where-Object { $_.Name -notin $Common } | Select-Object Name
        $CommandParameterNames = $CommandParameters.Name

        $Help = Get-Help $Command -ErrorAction SilentlyContinue
        $HelpParameters = $Help.Parameters.Parameter | Where-Object { $_.Name -notin $Common } | Sort-Object -Property Name -Unique
    }

    # If help is not found, synopsis in auto-generated help is the syntax diagram
    It 'Should not be auto-generated' {
        $Help.Synopsis | Should -Not -Contain 'CommonParameters'
    }

    # Should be a description for every function
    It 'Should have a description' {
        $Help.Description | Should -Not -BeNullOrEmpty
    }

    # Should be at least one example
    It 'Should have at least one example' {
        ($Help.Examples.Example | Select-Object -First 1).Code | Should -Not -BeNullOrEmpty
    }

    # Should be a description for every parameter
    It 'Should have description for parameter: <_>' -ForEach $CommandParameterNames {
        $CommandParameterName = $_
        $ParameterHelp = $HelpParameters | Where-Object { $_.Name -ieq $CommandParameterName }
        $ParameterHelp.Description.Text | Should -Not -BeNullOrEmpty
    }
}

Error

  [-] Should have description for parameter: <_> 23ms (19ms|4ms)
   CommandNotFoundException: The term 'PatchName' is not recognized as a name of a cmdlet, function, script file, or executable program.
   Check the spelling of the name, or if a path was included, verify that the path is correct and try again.
   at <ScriptBlock>, /MCS/Tests/Common/Help.Tests.ps1:15
  [-] Should have description for parameter: <_> 25ms (18ms|7ms)
   CommandNotFoundException: The term 'PatchVersion' is not recognized as a name of a cmdlet, function, script file, or executable program.
   Check the spelling of the name, or if a path was included, verify that the path is correct and try again.
   at <ScriptBlock>, /MCS/Tests/Common/Help.Tests.ps1:15
  [-] Should have description for parameter: <_> 25ms (21ms|4ms)
   CommandNotFoundException: The term 'ProductName' is not recognized as a name of a cmdlet, function, script file, or executable program.
   Check the spelling of the name, or if a path was included, verify that the path is correct and try again.
   at <ScriptBlock>, /MCS/Tests/Common/Help.Tests.ps1:15

[fflaten]

-ForEach/TestCases are evaluated during discovery, so you need to define the parameter-list in BeforeDiscovery. Related #1684

I'd also recommend to get function help once per command (Describe) only by using BeforeAll. BeforeEach runs before every test. Try:

BeforeDiscovery {
    $Cmdlets = Get-Command -Name Get-Item
}

Describe 'Help for <_>' -ForEach $Cmdlets {
    BeforeDiscovery {
        $Common = 'Debug', 'ErrorAction', 'ErrorVariable', 'InformationAction', 'InformationVariable', 'OutBuffer', 'OutVariable', 'PipelineVariable', 'Verbose', 'WarningAction', 'WarningVariable', 'Confirm', 'Whatif'

        $Command = $_ # Get current from -ForEach $Cmdlets in discovery-phase
        $CommandParameters = $Command.ParameterSets.Parameters | Sort-Object -Property Name -Unique | Where-Object { $_.Name -notin $Common } | Select-Object Name
        $CommandParameterNames = $CommandParameters.Name
    }

    BeforeAll {
        # $Common defined in BeforeDiscovery is not available in run-phase. Would have to pass it using `-Foreach` in a outer Describe to reuse.
        $Common = 'Debug', 'ErrorAction', 'ErrorVariable', 'InformationAction', 'InformationVariable', 'OutBuffer', 'OutVariable', 'PipelineVariable', 'Verbose', 'WarningAction', 'WarningVariable', 'Confirm', 'Whatif'

        $Command = $_ # Get current from -ForEach $Cmdlets in run-phase
        $Help = Get-Help $_ -ErrorAction SilentlyContinue
        $HelpParameters = $Help.Parameters.Parameter | Where-Object { $_.Name -notin $Common } | Sort-Object -Property Name -Unique
    }

    # Should be a description for every parameter
    It 'Should have description for parameter: <_>' -ForEach $CommandParameterNames {
        $CommandParameterName = $_
        $ParameterHelp = $HelpParameters | Where-Object { $_.Name -ieq $CommandParameterName }
        $ParameterHelp.Description.Text | Should -Not -BeNullOrEmpty
    }
}

[PleaseStopAsking]

Clearly I am not grasping each of the phases yet so I will continue to read up on the specific's of each. I tested your suggestion and it does indeed accomplish the goal intended. Can't thank you enough for pointing me in the right direction.