try catch with addError not showing message

Question

I wrote this code in a trigger so addError works fine, but when code fails within a try-catch block i expected my addError message but i getting a error as i not using the try-catch.

public with sharing class triggerHandler{

public static void afterUpdate(List<Opportunity> Trigger.new)
    CustomObject__c ob = [SELECT id, name, code__c from CustomObject__c where Opportunity__c In :(Trigger.new) ];
    ob.code__c = 12345;

    try{
        update ob;
    }catch(DmlException e){
        for(Oppotunity opp : Trigger.new){
          opp.addError('Show a error');
    }
  }
}

But i getting an error like: We hit a snag. Review the errors on this page. OppotunityTrigger: execution of AfterUpdate caused by: System.DmlException: Update failed. First exception on row 0 with id a0J1w000006574PEA2; first error: Your code number is wrong.

my issue is i expected "Show a error" message when i got a DmlException because is within catch but i getting the whole exception message.

sorry my english help please!!

Answer 1

This code fundamentally misunderstands the SObject.addError and Trigger.new variable access if this code is not in a trigger - these are only for use in a trigger. See this question for details.

Showing an error back to the user may be handled in various different ways. An example for Visualforce is to use ApexPages.addMessage. Your question doesn't provide the context, so I don't know if this is an appropriate approach.

If this code is in a trigger for Opportunity after update, then the opportunity object will be locked as you are in the after phase. You could try switching to before phase instead? Take a look at the trailhead for triggers.

Note, too, that you can use Database.update with allOrNothing as false if you want to allow some of the data to be updated whilst getting details of the errors for those objects that cannot be updated as part of the result details.