Could not think of any case where return parameter could have a default value, double checking with the area experts CC @AaronRobinsonMSFT @jkotas @steveharter

Tagging mono folks @lambdageek @marek-safar @vargaz

Why is the extra if needed?

Because GetDefaultValue returns null instead of DBNull.Value/Type.Missing, which makes HasDefaultValue return true for return parameters. Do we consider a return parameter in mono to have default values for a valuable reason or it is just an error ?

I think better fix would be to update add_parameter_object_to_array

I don't think it would fix the probem. The add_parameter_object_to_array processes only input parameters. In fact, The ReturnParameter is constructed here :

Mono does not provide DefaultValueImpl to RuntimeParameterInfo as you can see, we can add a new method in mono like get_retval_marshal that return a hardcoded DbNull.Value or do it directly in the dedicated constructor:

Let me know which one do you prefer

Setting the default DbNull.Value directly in the dedicated constructor sounds reasonable to me, ping @marek-safar for opinion