如何在ASP.NET的下拉列表更改时从数据库中获取数据，并将其显示为下载文件的链接？

// 数据库连接字符串
string connectionString = "your_connection_string";

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        // 绑定下拉列表的数据
        BindDropDownList();
    }
}

protected void ddl_SelectedIndexChanged(object sender, EventArgs e)
{
    // 获取选中的值
    string selectedValue = ddl.SelectedValue;

    // 根据选中的值从数据库中获取数据
    string data = GetDataFromDatabase(selectedValue);

    // 将数据写入响应流，并设置响应的Content-Disposition头部
    Response.Clear();
    Response.ContentType = "application/octet-stream";
    Response.AddHeader("Content-Disposition", "attachment; filename=download.txt");
    Response.Write(data);
    Response.End();
}

private void BindDropDownList()
{
    // 连接数据库并执行查询语句
    using (SqlConnection connection = new SqlConnection(connectionString))
    {
        string query = "SELECT Id, Name FROM YourTable";
        SqlCommand command = new SqlCommand(query, connection);
        connection.Open();
        SqlDataReader reader = command.ExecuteReader();

        // 绑定数据到下拉列表
        ddl.DataSource = reader;
        ddl.DataTextField = "Name";
        ddl.DataValueField = "Id";
        ddl.DataBind();

        reader.Close();
    }
}

private string GetDataFromDatabase(string selectedValue)
{
    // 连接数据库并执行查询语句
    using (SqlConnection connection = new SqlConnection(connectionString))
    {
        string query = "SELECT Data FROM YourTable WHERE Id = @Id";
        SqlCommand command = new SqlCommand(query, connection);
        command.Parameters.AddWithValue("@Id", selectedValue);
        connection.Open();
        string data = command.ExecuteScalar().ToString();

        return data;
    }
}