Tranfering data from richtetxbox to a listbox c#

If the items are listed on consecutive lines in the RTB, then try this (Windows Forms assumed):

        private void button1_Click(object sender, EventArgs e)
        {
            if (richTextBox1.Text.Length > 0)
            {
                listBox1.Items.AddRange(richTextBox1.Lines);
            }
        }

It's also assumed that, if there are existing items in the ListBox, these will be added to, not overwritten.