How To Refresh ComboBox ?

Question

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]);
            }

        }