How to get deselect text from a full text in a combobox

Question

I need to get read a deselect text from a full text in a combobox where some text are highlighted.
Suppose the searching text is emergency where emer is not highlighted but gency is highlighted.
Now I want to read this deselect text emer to perform operation.
 
textbox1: emergency (at typing)

textbox2.Text = textbox1.Text.Substring(textbox1.SelectionStart, textbox1.SelectionLength);

I am trying this code variously. But it's not working.

Help me plz.Thanks in advance.