When writing UBL files and `lineTotalAmount` was omitted, it causes a validation error, because `LineExtensionAmount` is missing

[Dirksche]

The description of the lineTotalAmount parameter of the function AddTradeLineItem states "This parameter is optional. If it is not filled, the line total amount is automatically calculated based on netUnitPrice and billedQuantity".

This is true for writing CII files, but not for writing UBL files.
When creating UBL files, the section </cac:InvoiceLine> is written without the compulsory <cbc:LineExtensionAmount> section. This leads to validation errors:

[stephanstapel]

Thanks for raising this.
There is a back and forth in the component if it should calculate anything or not.
In the past, I got a lot of complaints because things were calculated automatically but resulting values did not match to the values that were calculated in the source ERP system. Give me a moment to think about the pr.

[Dirksche]

@stephanstapel The lineTotalAmount parameter of the AddTradeLineItem function represents BT-131.
BT-131 is a required field:

In UBL files, the cbc:LineExtensionAmount section (which represents BT-131) is also a required section within the cac:InvoiceLine section:

UBL files with a cac:InvoiceLine section which does not contain a cbc:LineExtensionAmount section, do not validate against the schema and are therefore validated as invalid (see screenshot above).

So if anyone calls the AddTradeLineItem function without the lineTotalAmount parameter, I see three possibilities:

1. Current situation: Don't output the cbc:LineExtensionAmount section. --> This leads to an invalid e-invoice.
2. Error out/throw an exception. --> Not nice.
3. Calculate lineTotalAmount by using netUnitPrice and billedQuantity, as described in the documentation of the function and as it is already done for the CII format.

Having the same behaviour for CII and UBL also has the advantage, that it will produce same/similar x-invoices, when someone switches from CII to UBL or vice versa.