Dynamics 365 – Get E-mail Address of Activity Parties from SDK (RetrieveMultiple)

public List<string> RetrieveActivityPartyEmails(IOrganizationService service)
        {
            var activityId = new Guid("[your_activity_id]");
            List<string> emailAdresses = new List<string>();

            var activityParties = service.RetrieveMultiple(new QueryExpression("activityparty")
            {
                ColumnSet = new ColumnSet("addressused"),
                Criteria = new FilterExpression(LogicalOperator.And)
                {
                    Conditions =
                        {
                            new ConditionExpression("activityid", ConditionOperator.Equal, activityId)
                        }
                }
            }).Entities;

            for (int i = 0; i < activityParties.Count; i++)//addressused
            {
                var party = activityParties[i];

                if (party.Attributes.Contains("addressused"))
                    emailAdresses.Add(party.GetAttributeValue<string>("addressused"));
            }

            return emailAdresses;
        }