How to initialize child class using function that returns a parent class?

Question

I've extended the DataTable class as follows:

class DataTableExtended:DataTable
    {
        public void specialMethod()
        {}
    }

I want to parse an XML node into this child class, which I tried to do with the following:

public DataTableExtended parseNodeToDataTable()
        {
            DataSet ds = new DataSet();
            XmlNodeReader reader = new XmlNodeReader(this.resultNodes);
            ds.ReadXml(reader);
            DataTable dt = ds.Tables[1];
            DataTableExtended dte=(DataTableExtended) dt;
            return dt;
        }

It's throwing an InvalidCastException. From what I've read so far, this is because it's not possible to cast a parent class to a child class. Is that accurate? If so, I know I can rewrite the DataTableExtended constructor so that it accepts a DataTable argument and copies that table's information, but I was hoping that there's a more direct way of doing this.

score 2 · Accepted Answer · answered Sep 24 '12 at 21:38

I would write an extension method for DataTable instead of subclassing it

public static class DataTableExtensions
{
    public static void SpecialMethod(this DataTable dt)
    {
        //do something
    }
}

--

DataTable dt = .........
dt.SpecialMethod();

score 0 · Answer 2 · edited May 23 '17 at 10:24

0

I like to read the : operator as is a. So a DataTableExtended is a DataTable. Note that this does not imply that a DataTable is a DataTableExtended, hence the failure to cast.

I think the best way of "casting" in this direction is to take an instance of the base class in your constructor, as you suggest.

Also see this answer.

edited May 23 '17 at 10:24

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answered Sep 24 '12 at 21:37

RichardTowers

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How to initialize child class using function that returns a parent class?

2 Answers2