在数据库中插入下拉列表值

我有一个下拉列表，我已经绑定到一个表。问题是当我试图将dropDownlist控件的选定值插入数据库时，我没有得到正确的值，而是得到整数数据为2和1。

这是我的下拉列表控件:

<asp:DropDownList
  ID="ddlproductName"
  runat="server"
  DataTextField="Prod_Name"
  DataValueField= "Product_Id"
  >
</asp:DropDownList>

后台代码:

protected void Page_Load(object sender, EventArgs e)
{
  ddlproductName.SelectedIndex = 0;
  if (!IsPostBack)
  {
    tbDate.Text = DateTime.Today.ToShortDateString();
    string cs = ConfigurationManager.ConnectionStrings["DBCS"].ConnectionString;
    using (SqlConnection con = new SqlConnection(cs))
    {
      SqlCommand cmd = new SqlCommand("Select Product_Id,Prod_Name from Add_Product", con);
      con.Open();
      SqlDataReader rdr = cmd.ExecuteReader();
      ddlproductName.DataSource = rdr;
      ddlproductName.DataBind();
    }
  }
}
protected void btnPurchase_Click(object sender, EventArgs e)
{
  try
  {
    string custName = tbcustName.Text;
    string prodName = ddlproductName.SelectedValue;
    int quantity = Convert.ToInt32(tbQuantity.Text);
    string date = tbDate.Text;
    string voucherNo = tbVoucher.Text;
    double amount = Convert.ToDouble(tbAmount.Text);
    PurchProduct purch = new PurchProduct(custName, prodName, quantity, date, voucherNo, amount);
    ConnectionClass.purchaseProd(purch);
    LblPurchase.Text = "Submitted succesfully";
  }

和purchaseProd方法如下:

public static void purchaseProd(PurchProduct purchase)
{
  cmd = new SqlCommand("insert into TblPurchase(custName, prodName, quantity, date,voucherNo,amount) values (@custName, @prodName, @quantity, @date,@voucherNo,@amount)", con);
  cmd.Parameters.AddWithValue("@custName", purchase.custName);
  cmd.Parameters.AddWithValue("@prodName", purchase.prodName);
  cmd.Parameters.AddWithValue("@quantity", purchase.quantity);
  cmd.Parameters.AddWithValue("@date", purchase.date);
  cmd.Parameters.AddWithValue("@voucherNo", purchase.voucherNo);
  cmd.Parameters.AddWithValue("@amount",purchase.amount);
  try
  {
    con.Open();
    cmd.ExecuteNonQuery();
  }
  finally
  {
    con.Close();
    cmd.Parameters.Clear();
  }
}