Gets the number of selected items.
<p>
This operation will fail if the number of selected
items cannot be queried from the OS.
@return the number of selected items in the widget
@exception SWTException <ul>
<li>ERROR_THREAD_INVALID_ACCESS when called from the wrong thread</li>
<li>ERROR_WIDGET_DISPOSED when the widget has been disposed</li>
</ul>
Gets the number of selected items. <p> This operation will fail if the number of selected items cannot be queried from the OS.
@return the number of selected items in the widget
@exception SWTException <ul> <li>ERROR_THREAD_INVALID_ACCESS when called from the wrong thread</li> <li>ERROR_WIDGET_DISPOSED when the widget has been disposed</li> </ul>