SortableListBox.cs

// コード汚いです。。
// import System.Windows.Forms
public class SortableListBox : ListBox
{
    int moveItemIndex = -1;
    public event EventHandler ItemSorted; 

    public SortableListBox()
        : base()
    {
    }

    protected override void OnMouseDown(MouseEventArgs e)
    {
        base.OnMouseDown(e);

        if (e.Button == MouseButtons.Left)
        {
            moveItemIndex = this.IndexFromPoint(e.Location);
        }
    }

    protected override void OnMouseMove(MouseEventArgs e)
    {
        base.OnMouseMove(e);

        if (moveItemIndex >= 0)
        {
            int current = this.IndexFromPoint(e.Location);
            if (moveItemIndex != current && current >= 0)
            {
                object item = this.Items[moveItemIndex];
                this.Items.RemoveAt(moveItemIndex);
                this.Items.Insert(current, item);
                moveItemIndex = current;
                this.SelectedIndex = current;
            }
        }
    }

    protected override void OnMouseUp(MouseEventArgs e)
    {
        base.OnMouseUp(e);

        if (e.Button == MouseButtons.Left)
        {
            moveItemIndex = -1;
            this.OnItemSorted(EventArgs.Empty);
        }
    }

    protected virtual void OnItemSorted(EventArgs e)
    {
        if (ItemSorted != null) ItemSorted(this, EventArgs.Empty);
    }
}