Warn if Format command (Format-Table, etc.) are used in the middle of a pipeline.

IncorrectUseOfFormatCmdletsInMiddleOfPipeline.psm1

using namespace System.Management.Automation.Language
using namespace Microsoft.Windows.PowerShell.ScriptAnalyzer.Generic


function IncorrectUseOfFormatCmdletsInMiddleOfPipeline {
    <#
        .SYNOPSIS
        Do not place format-table in the middle of a pipeline
        .DESCRIPTION
        Detects when Format commands are commonly misused in the middle of a pipeline, causing unexpected results with display objects that dont have the same properties as the original piped object.
        .NOTES
        None
    #>
    [CmdletBinding()]
    [OutputType([DiagnosticRecord[]])]
    Param
    (
        [Parameter(Mandatory = $true)]
        [ValidateNotNullOrEmpty()]
        [System.Management.Automation.Language.ScriptBlockAst]
        $ScriptBlockAst
    )

    Process {
        trap {
            $PSCmdlet.ThrowTerminatingError($PSItem)
        }
        #TODO: Detect additional aliases automatically within the same script?
        $formatBadList = @(
            'Format-Table'
            'ft'
            'Format-List'
            'fl'
            'Format-Wide'
            'fw'
        )


        #Predicate that finds format commands not at the end of a pipeline
        $findFormatTable = {
            param ([Ast]$Ast)

            #We use a "disqualifying" approach to keep this performant.
            if ($Ast -isnot [CommandAst]) {return}
            [CommandAst]$commandAst = $Ast
            if ($commandAst.GetCommandName() -notin $formatBadList) {return}
            if ($commandAst.Parent -isnot [PipelineAst]) {return}
            [PipelineAst]$parent = $commandAst.Parent

            #If the command is not the last element of the pipeline, we have a match
            return $parent.PipelineElements[-1] -ne $commandAst
        }

        [DiagnosticRecord[]]$result = $ScriptBlockAst.FindAll($findFormatTable, $false) | ForEach-Object {
            #Emit a warning record for each format table found
            [DiagnosticRecord]@{
                Message  = "$($PSItem.GetCommandName()) should not be used in the middle of a pipeline. It outputs special display objects that are generally incompatible with further pipeline use. Consider using Select-Object instead"
                Extent   = $PSItem.Extent
                RuleName = $PSCmdlet.MyInvocation.InvocationName
                Severity = 'Warning'
            }
        }
        return $result
    }
}

Export-ModuleMember IncorrectUseOfFormatCmdletsInMiddleOfPipeline

PSScriptAnalyzerSettings.psd1

@{
    CustomRulePath = '.\IncorrectUseOfFormatCmdletsInMiddleOfPipeline.psm1'
    IncludeDefaultRules = $true
    IncludeRules = @(
        'IncorrectUseOfFormatCmdletsInMiddleOfPipeline'
    )
}

zDemo.ps1

# Should Warn
Get-Process | Format-Table | ForEach-Object { $_ } 

# Should Warn
Get-Process 
| Format-Table 
| ForEach-Object { $_ }

# Should Warn
Format-Wide
| Get-Process
| ForEach-Object { $_ }

#OK
Get-Process | ForEach-Object { $_ } | Format-Table

#OK
Get-Process
| ForEach-Object { $_ }
| Format-Table

#OK
Get-Process
| ForEach-Object { $_ }
| Format-Wide

Awesome work! 👍
Tip: You should change to $false in $ScriptBlockAst.FindAll($findFormatTable, $true) as $true will return duplicates. Nested scriptsblock (ex a function) will get it's own run through IncorrectUseOfFormatCmdletsInMiddleOfPipeline

Awesome work! 👍 Tip: You should change to $false in $ScriptBlockAst.FindAll($findFormatTable, $true) as $true will return duplicates. Nested scriptsblock (ex a function) will get it's own run through IncorrectUseOfFormatCmdletsInMiddleOfPipeline

Good tip! Wasn't sure if nested was required or not, thanks for confirming.