private void cmb_batch_SelectedIndexChanged(object sender, EventArgs e)
{
txt_batch_no.Text = General_methods.get_batch_no_from_batch_name(cmb_batch.Text);
txt_course_namer.Text = General_methods.get_course_name_from_course_no(General_methods.get_course_no_of_batch_from_batch_no(txt_batch_no.Text));
cmb_medium.Text = General_methods.get_medium_from_batch_no(txt_batch_no.Text);
stud_no_change();
}