Question

How it is posible to fix this script?

Yes, I´m changing the collection in the foreach loop and this is the reason for this error.

An error occurred while enumerating through a collection: Collection was modified; enumeration operation may not execute..
At C:\Users\user\Documents\PowerShell\ChangeAllListsV2.ps1:47 char:20
+ foreach <<<< ($list in $webLists)
+ CategoryInfo : InvalidOperation: (Microsoft.Share…on+SPEnumerator:SPEnumerator) [], RuntimeException
+ FullyQualifiedErrorId : BadEnumeration

#Script change in all lists the required field property "testfield" to false


#Part 0 - Configuration

$urlWebApp = "http://dev.sharepoint.com"
$countFound = 0
$countList = 0
$countFoundAndChange = 0

#Part 1 - PreScript  

$snapin = Get-PSSnapin | Where-Object {$_.Name -eq "Microsoft.SharePoint.Powershell"}

if ($snapin -eq $null)

{
    Write-Host “Loading SharePoint Powershell”
    Add-PSSnapin Microsoft.SharePoint.Powershell
}

#Part 2 - Script

$webApp = Get-SPWebApplication $urlWebApp

#$webApp | fl

    $webAppSites = $webApp.sites

    foreach($site in $webAppSites)
    {
        Write-Host "***********************************************************************"
        Write-Host "Found site: " $site -foreground blue

        $siteAllWebs = $site.AllWebs

        foreach($web in $siteAllWebs)
        {
            Write-Host "Found web: " $web -foreground blue
            #$web | fl

           $webLists = $web.Lists

            foreach($list in $webLists)
            {
             $countList ++

             Write-Host "Found list: " $list -foreground blue

                #Change list property

                $field = $Null
                $field = $list.Fields["testfield"]

                    if($field){
                    Write-Host "Field found: " $list -foreground green
                    #Write-Host "in web: " $web -foreground green
                    $countFound ++

                        try{

                            if($field.Required)
                            {

                            #######################################################
                            $field.Required = $False
                            $field.Update()
                            #######################################################

                            $field = $Null
                            Write-Host "Done!: Change list: " $list -foreground  green
                            $countFoundAndChange ++                    

                            }else{ 
                            Write-Host "Already!: Change list: " $list -foreground  green       

                            }

                        }
                        catch{
                            $field = $Null
                            Write-Host "Error!: Change list: " $list -foreground red
                            Write-Host "in web: " $web -foreground red
                            $_

                        }

                    }


            } 


        }


    }



Write-Host "Found lists: " $countList
Write-Host "Found lists with column [testfield]: " $countFound
Write-Host "Change lists with column [testfield]: " $countFoundAndChange

Answer 1

The SPListCollection tends to modify the collection when updating its properties (fields, event receivers, etc.). You can use a for-loop instead:

for ($i = 0; $i -lt $webLists.Count; $i++)
{
  $list = $web.Lists[$i];
  # ...
}