下拉列表不会按预期持续更新

Question

我希望当用户点击要更新的其他选择时。但事实并非如此。相反，我必须单击按钮重新加载页面才能这样做。我不确定为什么会这样或如何解决它。

public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        int ucost = int.Parse(DropDownList1.SelectedItem.Value);
        ttbox.Text = ucost.ToString();
    }

    protected void ListBox1_SelectedIndexChanged(object sender, EventArgs e)
    {

    }

    protected void Button1_Click(object sender, EventArgs e)
    {

    }

    protected void ttbox_TextChanged(object sender, EventArgs e)
    {
    }

    protected void DropDownList1_SelectedIndexChanged1(object sender, EventArgs e)
    {
    }
}