Looking for controller to send and reply on emails in Salesforce

I’m looking for some support when it comes to the development of a controller used to send email and also reply. Currently I have a controller to send email (from case) which works just fine. But I need a similar controller, preferable the same, I can use for replies making the reply to be linked to the case (ParentID) and also the original email (ParentId,ReplyToEmailMessageId)

Currently when calling the reply I get “Conversion from Runtime Type EmailMessage to Case” error.