如何使用asp.net根据dropdownlist项值的选择在表单中添加更多字段

当我在dropdownlist中选择一个值，然后在同一页面上显示/插入更多字段，如：在plan type value中，选择MNP，然后显示要插入的字段PortNumber、PortId，当选择NC，然后显示要插入的其他字段，依此类推。
我不知道怎么做 我的代码是：

    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;

    public partial class Caller : System.Web.UI.Page
    {
    SqlConnection con;
    SqlCommand cmd;
    protected void Page_Load(object sender, EventArgs e)
    {
        con = new SqlConnection();
       con.ConnectionString=@"Data Source=.\SQLEXPRESS;AttachDbFilename=F:\Asp      Program\LMS\App_Data\LMS.mdf;Integrated Security=True;User Instance=True";
       con.Open();
     }
    protected void Button1_Click(object sender, EventArgs e)
    {

        String CName, CAddress,PlanType, CNumber, CAltNumber,WNumber;

        CName = TextBox1.Text;
        CNumber = TextBox2.Text;
        CAltNumber = TextBox3.Text;
        PlanType = DropDownList1.SelectedValue;
        WNumber = TextBox4.Text;
        CAddress = TextBox5.Text;
        string Date = (Convert.ToDateTime(TextBox6.Text)).ToShortDateString();
        cmd = new SqlCommand();
        cmd.Connection = con;
        cmd.CommandText = "insert into CustomerDetails values(@CName,@CNumber,@CAltNumber,@PlanType,@WNumber,@CAddress,@Date)";
        cmd.Parameters.AddWithValue("@CName", CName);
        cmd.Parameters.AddWithValue("@CNumber", CNumber);
        cmd.Parameters.AddWithValue("@CAltNumber", CAltNumber);
        cmd.Parameters.AddWithValue("@PlanType", PlanType);
        cmd.Parameters.AddWithValue("@WNumber", WNumber);
         cmd.Parameters.AddWithValue("@CAddress", CAddress);
         cmd.Parameters.AddWithValue("@Date",Date);

        int res = cmd.ExecuteNonQuery();
        if (res != 0)
        {
            Label8.Text = "Details Submitted.";
            TextBox1.Text = "";
            TextBox2.Text = "";
            TextBox3.Text = "";
            TextBox4.Text = "";
            TextBox5.Text = "";
            TextBox6.Text = "";
            TextBox1.Focus();
        }
        else
            Label8.Text = "Please Check Details And Insert Again...";
   }
    protected void TextBox1_TextChanged(object sender, EventArgs e)
    {

    }

}

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您可以在dropdown selected Indexchange事件中编写代码。好的，但我只是初学者，所以不知道如何编写，请帮助在selected Indexchange事件中需要检查dropdown selected index，并在上面显示或隐藏控件