I tried $query.ForEach({$fileMapping.Add($_.n, $_.f)})  and it worked perfectly.
Thank you very much.

I'm trying to pass  the values of  this hash table into  "restore-dbaDatabase"
Restore-DbaDatabase -SqlInstance $instanceName -Path $backupPath -WithReplace -MaintenanceSolutionBackup -DatabaseName $Databasename -FileMapping $FileMapping -OutputScriptOnly |Out-File $File
so I was...