How do I pass an array of an extended type in codesys/twincat3?

Say I have an abstract function block AValve that I extend for various types of valve. I extend that AValve in order to implement it as a BasicValve. Also I have a function block that takes an array of AValve, which looks like this

FUNCTION_BLOCK ValveDispatch
VAR_IN_OUT
   valves : ARRAY[*] OF AVALVE;
END_VAR

If I try to pass an array of BasicValve into this function block, I'm met with:

Cannot convert type 'ARRAY [0..5] OF BasicValve' to type 'ARRAY [*] OF AVALVE' of VAR_IN_OUT 'valves'

Thinking that maybe codesys just couldn't handle both extended types AND variable length arrays at the same time, I've tried doing a set length array as an input, just for testing as I need the variable length. Doing so gives a slightly different error that seems to mean the same thing:

Type `ARRAY[0..5] of BasicValve' is not equal to type 'ARRAY [0..5] OF AVALVE' of VAR_IN_OUT 'valves'

Is there a way I can make this work? Passing a single extended object into an input expecting its base type works fine, but doing so with arrays seems to be unsupported.

Solution

The issue you are seeing actually comes down to a pretty simple point, arrays are value datatypes.

That is, when you are trying to pass ARRAY [*] OF FB_Ext through the VAR_IN_OUT what you are actually doing is creating a whole new datatype ARRAY [*] OF FB_Ext which does not extend ARRAY [*] OF FB_Base.

IF you look at an array at the bit level in ST the issue becomes pretty clear, an array itself has a size of ElementSize * ElementCount, So trying to force a bigger datatype FB_Ext into the smaller dataspace of FB_Base is not going to work without messing everything up.

So you will have to do this handling with either pointers or interfaces.

TLDR: What you want to do is not possible in ST.

So this was originally going to be a comment, but it got a bit beyond the size limit. Posted as a wiki instead.