Excel-DNA has built-in support only for .NET types that match types natively supported by the Excel C API. To support additional data types there needs to be some conversion - in this case from
List to an
object[,] array. You can either do the conversion yourself, or you can use the Excel-DNA Registration helper library to register a conversion that will by applied at runtime. You could also implement a generic conversion that uses reflection to expose all the properties of your class, or something like that. But there is nothing like this 'in the box' currently.
For general Excel-DNA usage questions like this, the Excel-DNA Google group is best.