Question

I am trying to create a parser framework for XML strings containing SQL query results. The intent is to inherit from generic classes, which are instantiated with the column data types. The included code is for the single-column variety. There will be additional classes for two columns, etc.

I need to be able to specify that the generic type must support the Parse(string) method. How do I do this?

abstract class OneColumnParser<Col1>
{
    abstract string Column1;

    List<Col1> ParseQueryResult(string queryResult)
    {
        XmlDocument xDoc = new XmlDocument();
        xDoc.LoadXml(queryResult);
        List<Col1> results = new List<Col1>();

        foreach (XmlNode xNode in xDoc.GetElementsByTagName(Column1))
        {
            results.Add(Col1.Parse(xNode.InnerText));
        }
    }
}

When I compile the above, I get “‘Col1’ is a ‘type parameter’, which is not valid in the given context” on the results.Add() line, because I haven’t specified that the type must support the method. But how?

Answer 1

Because interfaces can’t have static methods, you can’t (directly) do what you’re asking. Reflection is one way of solving the problem, but it’s only verified at runtime, not enforced by the compiler. E.g.

abstract class OneColumnParser<TCol>
{
    private static MethodInfo ParseInfo = typeof(TCol).GetMethod("Parse", BindingFlags.Public | BindingFlags.Static, null, new[] { typeof(string) }, null);
    abstract string Column1;

    static OneColumnParser()
    {
        if (typeof(TCol) != typeof(string) && (ParseInfo == null || !typeof(TCol).IsAssignableFrom(ParseInfo.ReturnType)))
            throw new InvalidOperationException("Invalid type, must contain public static TCol Parse(string)");
    }

    private static TCol Parse(string value)
    {
        if (typeof(TCol) == typeof(string))
            return (TCol)(object)value;
        else
            return (TCol)ParseInfo.Invoke(null, new[] { value });
    }

    public List<TCol> ParseQueryResult(string queryResult)
    {
        XmlDocument xDoc = new XmlDocument();
        xDoc.LoadXml(queryResult);
        List<TCol> results = new List<TCol>();

        foreach (XmlNode xNode in xDoc.GetElementsByTagName(Column1))
        {
            results.Add(Parse(xNode.InnerText));
        }

        return results;
    }
}

Unlike defining your own interface, this will work on existing types with Parse methods, such as int and DateTime. Update added code so that it will work on string as well.