Reterive all Activities in MS CRM 4.0 through QueryExpression object

Database Query: "SELECT * FROM ActivityPointerBase"

static void GetAllActivities()

        {

            CRMHelper objCRMHelper = new CRMHelper();

            CrmService service = objCRMHelper.GetCRMService();

            AllColumns activityPointerBaseColumns = new AllColumns();

            QueryExpression query = new QueryExpression();

            query.ColumnSet = activityPointerBaseColumns;

            query.EntityName = EntityName.activitypointer.ToString();

            BusinessEntityCollection result = service.RetrieveMultiple(query);

            if (result.BusinessEntities.Length > 0)

            {

                foreach (BusinessEntity objBE in result.BusinessEntities)

                {

                }

            }

        }