Adding more than 1 Linked Entity to a C# plugin

QueryExpression query = new QueryExpression();

query.EntityName = "invoicedetail";
query.ColumnSet.Columns.Add("f1_agreementinvoiceproduct");

query.LinkEntities.Add(new LinkEntity("invoicedetail", "invoice", "invoiceid", "invoiceid", JoinOperator.Inner));
query.LinkEntities[0].EntityAlias = "invoice";