How To Refresh ComboBox ?

Hi, I want to refresh the combobox. when i update data in child form, the combobox will be update in parent form. i tried following code it will update, but in combobox it also displayed the previous data.

so help me how can i refresh combobox ?

This is my code....

private void StudentRegistration_Load(object sender, EventArgs e)
{
cmbclass.Items.Clear();
FillClass();
}
public void FillClass()
{
SqlCommand cmdfill = new SqlCommand("select Class from AddClass", con);
SqlDataReader drfill = cmdfill.ExecuteReader();
while (drfill.Read())
{
cmbclass.Items.Add(drfill[0]);
}
}