Fill multiple textboxes after selecting two columns DropDownList in ASP.NET

I have a DropDownList which contains the two columns one is CardCode and other is CardName and that is linked to a SQL database. It currently shows a list of CardCode + List of Cardname. I am trying to make it so that once a CardCode + CardName is selected from the two columns dropdownlist, multiple textboxes are automatically filled (such as CardNum, CntctPerson,ListNum etc). I am able to automatically fill the data from selecting only CardCode now I want to show the related row to the CardCode + CardName dropdown list, I do not know how to fill the other rows by selecting 2 column's dropdown list(CardCode + CardName) .How can i do this? Thanks In Advance

Here is my aspx.cs Code below

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;
using System.Data.SqlClient;


namespace StackOver
{
    public partial class _Default : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            if (!Page.IsPostBack)
            {
                LoadOptions();
            }
        }
        protected void LoadOptions()
        {
            DataTable CardCode = new DataTable();

            SqlConnection connection = new SqlConnection("my connection string");
            using (connection)
            {
                SqlDataAdapter adapter = new SqlDataAdapter("SELECT CardCode,CardName, Address, CntctPrsn FROM OCRD", connection);

                adapter.Fill(CardCode);
                DropDownList1.DataValueField = "CardCode";
                DropDownList1.DataTextField = "CardCode";
                DropDownList1.DataBind();
            }
        }

       protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
        {


            string selected = DropDownList1.SelectedItem.Value;
            SqlConnection connection = new SqlConnection("my connection string");
            using (connection)
            {
                SqlCommand theCommand = new SqlCommand("SELECT CardCode, CardName, Address, CntctPrsn FROM OCRD WHERE CardCode = @CardCode", connection);
                connection.Open();
                theCommand.Parameters.AddWithValue("@CardCode", selected);
                theCommand.CommandType = CommandType.Text;

                SqlDataReader theReader = theCommand.ExecuteReader();

                   if (theReader.Read())
                    {
                        // Get the first row
                      // theReader.Read();

                        // Set the text box values
                        this.TextBox1.Text = theReader.GetString(0);
                        this.TextBox2.Text = theReader.GetString(1);
                        this.TextBox3.Text = theReader.GetString(2);
                     //   this.TextBox3 = reader.IsDBNull(TextBox3Index) ? null : reader.GetInt32(TextBox3Index)
                        // GenreID = reader.IsDBNull(genreIDIndex) ? null : reader.GetInt32(genreIDIndex)
                        this.TextBox4.Text = theReader.GetString(3);
                        //  TextBox5.Text = theReader.GetString(4);
                        //  TextBox6.Text = theReader.GetString(5);
                        //  TextBox7.Text = theReader.GetString(6);

                    }
                    connection.Close();
                }

            }



       public object TextBox3Index { get; set; }
    }

}

And also this is my .aspx code

<%@ Page Title="Home Page" Language="C#" MasterPageFile="~/Site.master" AutoEventWireup="true"
    CodeBehind="Default.aspx.cs" Inherits="StackOver._Default" %>

<asp:Content ID="HeaderContent" runat="server" ContentPlaceHolderID="HeadContent">
</asp:Content>
<asp:Content ID="BodyContent" runat="server" ContentPlaceHolderID="MainContent">
    <h2>
        Welcome to ASP.NET!
        <asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True" 
            DataSourceID="SqlDataSource1" DataTextField="CardCode" 
            DataValueField="CardName" 
            onselectedindexchanged="DropDownList1_SelectedIndexChanged">
        </asp:DropDownList>
        <asp:SqlDataSource ID="SqlDataSource1" runat="server" 
            ConnectionString="<%$ ConnectionStrings:myconnectionstring %>" 
            SelectCommand="SELECT [CardCode] + '----' + [CardName] as CardCode, CardName,[Address], [CntctPrsn] FROM [OCRD]">
        </asp:SqlDataSource>
    </h2>

           <br />
        <br />
    <p>
        &nbsp;</p>
    <p>
        &nbsp;</p>
    <p>
    Business Partner Code :&nbsp; 
        <asp:TextBox ID="TextBox1" runat="server" Width="192px" ></asp:TextBox>
    </p>
    <p>
    Business Partner Name :
        <asp:TextBox ID="TextBox2" runat="server" Width="192px" ></asp:TextBox>
    </p>
    <p>
     Address :&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 
        <asp:TextBox ID="TextBox3" runat="server" Width="196px" ></asp:TextBox>
    </p>

    <p>
    Contact Person Name :&nbsp; 
        <asp:TextBox ID="TextBox4" runat="server" Width="196px" ></asp:TextBox>
    </p>

     <p>
        <asp:TextBox ID="TextBox5" runat="server" ></asp:TextBox>
    </p>
     <p>
        <asp:TextBox ID="TextBox6" runat="server" ></asp:TextBox>
    </p>
     <p>
        <asp:TextBox ID="TextBox7" runat="server" ></asp:TextBox>
    </p>
</asp:Content>