// the abstract class used for any items we want to store in the extended list box :
public abstract class IExtendedListBoxItem : IComparable<IExtendedListBoxItem>
{
    // a value used to sort various items types
    public int SortingValue = -1;

    // implement the IComparable.CompareTo method
    public int CompareTo(IExtendedListBoxItem obj)
    {
        return (obj.SortingValue - this.SortingValue);
    }

    // ... the rest of the code here
}


// the extended list box
public partial class ExtendedListBox : ListBox
{
    protected override void Sort()
    {
        if (this.Items.Count <= 1)
            return;

        // extract all the elements to an array
        IExtendedListBoxItem[] items = new IExtendedListBoxItem[this.Items.Count];
        this.Items.CopyTo(items, 0);

        // sort the array
        Array.Sort(items);

        // clear the items' list
        this.Items.Clear();

        // add the sorted elements
        foreach (Object obj in items)
            this.Items.Add(obj);
    }

    // ... the rest of the code here
}

public abstract class IExtendedListBoxItem{    public override string ToString()    {        return this.SortingValue;    }    // the rest of the code ...}