how to add RequiredAttendees to Appointment in crm 2011

Question

I'm trying to add account to RequiredAttendees attribute of an appointment. My code works perfectly on email, fax, letter. But on appointment when is created, doesn't add the the attribute.

 string[] ToArr = new string[1];
            Guid To = Guid.Empty;
            try
            {
                if (Utility.To.Contains(";"))
                {
                    ToArr = Utility.To.Split(';');
                }
                else
                {
                    ToArr[0] = Utility.To;
                }

                EntityCollection collToParty = new EntityCollection();
                for (int i = 0; i < ToArr.Length; i++)
                {
                    if (Guid.TryParse(ToArr[i].ToString(), out To))
                    {

                        EntityReference to = new EntityReference(Utility.Toguidentityname, To);
                        Entity toParty = new Entity("activityparty");
                        toParty.Attributes.Add("partyid", to);
                        collToParty.EntityName = Utility.Toguidentityname;
                        collToParty.Entities.Add(toParty);
                    }
                    else
                    {
                        throw new Exception("Il valore del campo della stringa " + To + " toGuid non è un GUID:" + ToArr[i].ToString());

                    }
                }

                if (entity.LogicalName.Equals("appointment"))
                    entity.Attributes.Add("RequiredAttendees", collToParty);
                else
                    entity.Attributes.Add("to", collToParty);

            }
            catch (Exception ex)
            {
                LoggerObj.writeLog("Errore:" + ex.Message);
                throw;
            }

Answer 1

try to lowercase RequiredAttendees:

entity.Attributes.Add("requiredattendees", collToParty);