I have a requirement in which when click on delete button on the account it should allow you to delete only when its related contact status is inactive.
I have written the following code which gives me list of contacts per account..

    public class ContactsPerAccount{
    
        public static void contactResult()
        {
            Id accountId;
            List<Contact> contacts = new List<Contact>([SELECT Id, Name,Inactive__c FROM Contact WHERE AccountId = :accountId]);
            System.debug(contacts);
            for(Contact c:Contacts){
                if(c.Inactive__c=Boolean.valueOf('true') )
                {
                    System.debug('You Can not delete this account as its related contacts are Active');
                }
                else{
                    System.debug('Go ahead ');
                }
            }
        }
    }

If the `Account` contains any `Contact` whose `Inactive` flag is `false` it should throw an error message like

> 'You Can not delete this account as its related contacts are Active'...

In this case I have written a class here and I will use this class in trigger where I will use afterdelete event..
Can someone suggest how do I display an error message on UI after clicking on delete button..What else code do I need to modify?
is it possible to write a separate class that I written and call it in trigger?