Display All as Default value in ComboBox using C# and VB.Net in Windows Application

It displays blank on combobox and list all and other items. I need to display all as first item on combobox

        public void test()
        {
            DataTable dt1= new DataTable();
            dt1 = d.test(infodisplay);
            if (dt1.Rows.Count > 0)
            {
                DataRow dr = dt1.NewRow();
                dr[0] = 0;
                dr[1] = "All";
                dt1.Rows.InsertAt(dr, 0);

                cmbRack.DataSource = dt1;
                cmbRack.DisplayMember = "Rack";
                cmbRack.ValueMember="id";
                //cmbRack.SelectedIndex =0;
            }
        }

        public DataTable test(DisplayInfo dispinfo)
        {
            DataTable dt = new DataTable();
            try
            {
                if (sqlcon.State == ConnectionState.Closed)
                {
                    sqlcon.Open();
                }
                SqlCommand sccmd = new SqlCommand("Select id,Rack  from display ", sqlcon);
                

                SqlDataAdapter da = new SqlDataAdapter(sccmd);
                da.Fill(dt);
            }
            catch
            {

            }
            return dt;
        }