Add Code and decode to listbox

[Simon]

Newbie alert!
I have a webform listbox with what I am sure is a common requirement. I wish
to store a list of values but display a "translation" or decode. An example
would be 1, 2, 3, 4 and One, Two, Three, Four

I created

class ListBoxSpecial
{
public string Code;
public string Decode;

public override string ToString()
{
return Decode;
}

public ListBoxSpecial (string Code, string Decode)
{
this.Code = Code;
this.Decode = Decode;
}

I then populate the listbox, ListBox1 on page load. E.g. I loop round doing

ListBox1.Items.Add(new ListBoxSpecial(myCode, MyDecode)

I was hoping the ListBox would display
One
Two
Three
Four

but when selected I could return "2" or whatever code/decode was
appropriate. However when I try this I get the error
The best overloaded method match for
'System.Web.UI.WebControls.ListItemCollection.Add( string)' has some invalid
arguments
Argument '1': cannot convert from 'myproj.WebForm1.ListBoxSpecial' to
'string'

Any help would be gratefully received.

Cheers!
Simon