By using below code you can retrieve all Required and Optional Attendees of an Activity. You don’t need to specify the name of the entity. By doing it so, it allows you to use it for any of the activity entity.
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;
}
Crm Minds 