Adding a value to a picklist field of an entity in an org via apex

Question

All, I am trying to add a value to a picklist field of an entity (i.e. Document Attachment Status of Care Request Extension ) via apex class in my org only. I tried following approaches but those are not working: How to add new values to a picklist from apex code How can I add a new value to a picklist using APEX? Is there a proper documentation or any API for the above purpose?

Below is the code snippet of the approach which is I am trying right now, It is getting compiled successfully but not updating the picklist value of CaseSubStatus field of CareRequestExtension:

public void UpdatePicklistField(){
        MetadataService.MetadataPort service = createService();    
        MetadataService.CustomField customField = new MetadataService.CustomField();
        customField.fullName = 'CareRequestExtension.CaseSubStatus';       // your object. fields name
        customField.label = 'CaseSubStatus';                 // label of field.
        customField.type_x = 'Picklist';               // type
        metadataservice.ValueSet two = new metadataservice.ValueSet(); //new value
        two.valueSetName = 'second'; 
        customField.valueSet = two;
        List<MetadataService.SaveResult> results = service.createMetadata(new MetadataService.Metadata[] { customField });              
    }
    public MetadataService.MetadataPort createService(){
        MetadataService.MetadataPort service = new MetadataService.MetadataPort();
        service.SessionHeader = new MetadataService.SessionHeader_element();
        service.SessionHeader.sessionId = UserInfo.getSessionId();
        return service;    
    }

Answer 1

You are calling the createMetadata method, not updateMetadata and also not handling the result.

I suggest you follow the approach shared in this answer -

MetadataService API custom field label update

If you handle the result properly you will have very specific reason why you are not able to update or create the picklist.