对SelectedIndex更改启用文本框

我正在从数据库中获取一些记录，这取决于页面的id号，将显示的记录数量会有所不同，从1条记录到50条。我需要获取这些记录，然后选择要修改、删除或根本不更改的记录。我希望它看起来像这样：

无法使用stackoverflow图像上传器显示图像，因此在此处：https://dl.dropboxusercontent.com/u/9446763/code/dropdownlist.jpg

如果选择"无更改或删除"，我希望禁用文本字段，如果选择"修改"选项，我希望启用字段进行修改。

以下是我到目前为止所做的，我正在努力的部分是所选的索引更改，我不知道如何对其进行编码，这样当在下拉框中选择"修改"时，相应的文本框就会启用

ASPX页面

<asp:PlaceHolder ID="PlaceHolder1" runat="server"></asp:PlaceHolder>

CS页面

private void createControls()
    {
        var id = Request.Params["ID"];
        System.Data.OleDb.OleDbConnection pcn;
        System.Data.OleDb.OleDbCommand pcm;
        System.Data.OleDb.OleDbDataReader prs;
        pcn = new System.Data.OleDb.OleDbConnection("");
        pcm = new System.Data.OleDb.OleDbCommand();
        pcn.Open();
        pcm.Connection = pcn;
        var tableSql = @"select * FROM grouplist where ptid = '" + id + "'";
        pcm.CommandText = tableSql;
        prs = pcm.ExecuteReader();
        var rowcount = 0;
        while (prs.Read())
        {
            rowcount++; 
            PlaceHolder1.Controls.Add(new Literal() { Text = "<div class='row'><div class='span3'>" });
            TextBox tx = new TextBox();
            tx.ID = "txtData" + rowcount.ToString();
            tx.Text = prs["name"].ToString().Trim();
            tx.Width = 200;
            tx.CssClass = "span2";
            tx.Enabled = false;
            PlaceHolder1.Controls.Add(tx);
            PlaceHolder1.Controls.Add(new Literal() { Text = "</div><div class='span2'>" });
            TextBox txa = new TextBox();
            txa.ID = "amtData" + rowcount.ToString();
            txa.Text = prs["amt"].ToString();
            txa.CssClass = "span2";
            txa.Enabled = false;
            PlaceHolder1.Controls.Add(txa);
            PlaceHolder1.Controls.Add(new Literal() { Text = "</div><div class='span3'>" });
            DropDownList ddl = new DropDownList();
            ddl.Items.Add("No Change");
            ddl.Items.Add("Modify");
            ddl.Items.Add("Remove");
            ddl.Width = 200;
            ddl.CssClass = "span2";
            ddl.SelectedIndexChanged += new EventHandler(ddl_SelectedIndexChanged);
            ddl.AutoPostBack = true;
            ddl.ID = "ddlData" + rowcount.ToString();
            PlaceHolder1.Controls.Add(ddl);
            PlaceHolder1.Controls.Add(new Literal() { Text = "</div></div>" });
        }
        prs.Close();
    }
  void ddl_SelectedIndexChanged(object sender, EventArgs e)
    {
    }

public class YourClass {
    private DropDownList ddl;
    private TestBox txa;
    private void createControls {
        // ...
        TextBox txa = new TextBox();
        txa.ID = "amtData" + rowcount.ToString();
        // ...
        DropDownList ddl = new DropDownList();
        ddl.Items.Add("No Change");
        // ... etc.
    }
    void ddl_SelectedIndexChanged(object sender, EventArgs e)
    {
        if (ddl.SelectedIndex == 1)
            txa.Enabled = true;
    }
}