private void GetProduct()
{
DataTable dt = new DataTable();
dt = invoiceBLL.GetProduct();
cmbProduct.DataSource = dt;
dt.Rows.Add(new object[] { 0, "Select" });
dt.DefaultView.Sort = "ProductID ASC";
cmbProduct.ValueMember = "ProductID";
cmbProduct.DisplayMember = "ProductName";
cmbProduct.SelectedValue = 0;
}