Some custom Dynamo nodes need to handle inputs and outputs of variable nesting length. In python this can be done using var[]..[], but how to do it in C#?

To achieve this on the output of a node it is very easy as it’s done automatically when using the MultiReturn attribute, see the docs.

On the inputs, instead, the secret is to use the [ArbitraryDimensionArrayImport] object item attribute, see a sample node.

public static IList AddItemToEnd([ArbitraryDimensionArrayImport] object item, IList list)
        {
            return new ArrayList(list) //Clone original list
            {
                item //Add item to the end of cloned list.
            };
        }

Thanks to Peter for revealing this!