Retrieving a wall's Function Parameter (BuiltInParameter.FUNCTION_PARAM)

For part of a large Revit material-gathering project I'm working on, I need to retrieve a wall's function. Walls can be assigned six functions from within their type properties:

• Interior
• Exterior
• Foundation
• Retaining
• Soffit
• Core-Shaft

At first I thought it would be fairly straight-forward, and I'd just do this grab the valuestring of the 'FUNCTION_PARAM' like so:

eWall.get_Parameter("Function").AsValueString()

But that gave me a NULL value and crashed Revit. The approach that worked for me was this:

static public string IsExterior(Element e, Document doc)
{
    ElementId id = e.GetTypeId();
    ElementType wallType = doc.get_Element(id) as ElementType;

    Parameter wallFunction = wallType.get_Parameter
        (BuiltInParameter.FUNCTION_PARAM);
    WallFunction value  = (WallFunction)wallFunction.AsInteger();

    return value.ToString();
}

Thanks to Jeremy Tammik's SDK example for setting me straight.