An invoice merge is an implicit merge print. The head or parent data contains one record for each document which is linked 1:n with the detail or child data. To design and to print such a document the parent table has to be passed to List & Label by the DataMember property. Furthermore, the AutoMasterMode property has to be set to AsVariables as shown in the following example:
ListLabel LL = new
ListLabel();
LL.DataSource = CreateDataSet();
// Order data as variables
LL.DataMember = "InvoiceHeader";
LL.AutoMasterMode = LlAutoMasterMode.AsVariables;
// Call
Designer
LL.Design();
LL.Print();
LL.Dispose();