Designing a generic data class

Question

Hi All,

I want to popuate a listbox with objects of different types. I display the type (humanized) of the object in the listbox.

I created a class ListBoxView that overrides the ToString method and returns a string according to the type. I create a List of ListBoxView and databind this to the listbox.

public class ListBoxView
{
    private object m_value;

    public ListBoxView(Object value)
    {
        m_value = value;
    }

    public override string ToString()
    {
        if (m_value is Car)
            return "Car";
        else if (m_value is User)
            return "Human";
        else
            return "Not defined: " + m_value.GetType().ToString();
    }

    public T Value
    {
        get { return m_value; }
        set { m_value = value; }
    }
}

Can this better be solved with generics? Since I'm new to generics I'm having difficulties implementing using it.

Thanks

UPDATE The classes like Human and Car are part of a datamodel I can't change. I need to solve this on the user interface side

Answer 1

Actually I think I'd just implement ToString() on Car, Human etc and not bother with this class at all. Otherwise you'll have to update this class every time you add a new type.

If you're worried about I18n of the type names, then keep this class, but only so you can put the results of ToString() through a search on a resource file for the "localised" version of the name.

Answer 2

One very short solution, but maybe not the best, is to move the logic from your ToString method to the listBox1_Format event handler.

private string FormatForListbox(object m_value)
{
    if (m_value is Car)
        return "Car";
    else if (m_value is User)
        return "Human";
    else
        return "Not defined: " + m_value.GetType().ToString();
}

private void listBox1_Format(object sender, ListControlConvertEventArgs e)
{
   e.Value = FormatForListbox(e.ListItem);
}

Answer 3

Your solution scales poorly. In particular, look what happens if you want to handle special cases in ToString for other types than just Car and User.

You should follow Neil's advice to put this kind of logic elsewhere, namely in the specific classes – either as the ToString method or as a different method altogether (see code). In summary, using generics here won't help.

interface IInfoProvider {
    String Info { get; }
}

class User : IInfoProvider {
    …
    public override String Info { get { return "Human"; } }
}

class ListBoxView {
    …
    public override string ToString()
    {
        IInfoProvider infovalue = m_value as IInfoProvider;
        if (infovalue != null)
            return infovalue.Info;
        else
            return "Not defined: " + m_value.GetType().ToString();
    }
}