Clear all items that was selected from dropdown list

I have dropdown box with list of predifined items, from which I can select items into a text field with an add and remove buttons. these all work very nice.

Currently I have to remove one by one item incase I made some wrong selections via the remove button. You need to select the item and then click remove to have the item remove.

the section under the remove button is as below and work 100%

form1.ListSAPApplication.deleteItem(form1.ListSAPApplication.SelectedIndex);

I would now like to add another button to "clear all" selections from the text field without the necessity to first select the item. When I click CLEAR ALL button all of the selections should be cleared..

any idea how to write the statement to clear ALL selections from text box.

Thank you kindly.