At last i have done it using SelectionChanged
and creating SelectionChangedEventHandler
.
The code is as below (may be useful for some future user):
cb.SelectionChanged += new SelectionChangedEventHandler(comboBox1_SelectionChanged);
in converter()
function and outside converter function :
void comboBox1_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
txtblk2.Text = cb.SelectedValue.ToString() + " millions";
}