Hi, i have to change the code just a little bit but it finally worked:
var QEproduct_entitlementproducts_entitlement_entitlementid = "f582dc51-dd4c-e911-a977-002248010929";
// Instantiate QueryExpression QEproduct
var QEproduct = new QueryExpression("product");
QEproduct.Distinct = true;
// Add columns to QEproduct.ColumnSet
QEproduct.ColumnSet.AddColumns("name", "description");
QEproduct.AddOrder("productnumber", OrderType.Ascending);
// Add link-entity QEproduct_entitlementproducts
var QEproduct_entitlementproducts = QEproduct.AddLink("entitlementproducts", "productid", "productid");
// Add link-entity QEproduct_entitlementproducts_entitlement
var QEproduct_entitlementproducts_entitlement = QEproduct_entitlementproducts.AddLink("entitlement", "entitlementid", "entitlementid");
QEproduct_entitlementproducts_entitlement.EntityAlias = "aa";
// Define filter QEproduct_entitlementproducts_entitlement.LinkCriteria
QEproduct_entitlementproducts_entitlement.LinkCriteria.AddCondition("entitlementid", ConditionOperator.Equal, QEproduct_entitlementproducts_entitlement_entitlementid);
var results = service.RetrieveMultiple(QEproduct);
Thank you very much Kokulan.