public partial class WebForm1 : System.Web.UI.Page
    {
        string test;
        protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                DropDownList.Items.Add("test1");
                DropDownList.Items.Add("test2");
                test = "hello";
            }
        }
        protected void DropDownList_SelectedIndexChanged(object sender, EventArgs e)
        {
            Response.Write(test);//此时test为空值。。请问如何才能使得只初始化一次，而且又能让test持久化保存，如果我所需要持久化的变量不仅仅只是一个字符串变量，也可能是一个List<string>或者Dictionary类型，那么有哪些方法可以在服务器端只初始化一次，然后能够持久化保存呢？
        }
    }

 protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            DropDownList1.Items.Add("test1");
            DropDownList1.Items.Add("test2");
            test = "hello";
            ViewState["value"] = test;
        }
    }

    protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {
        if (ViewState["value"] != null)
        {
            Response.Write(ViewState["value"].ToString());
        }
        else
        {
            Response.Write("null");
        }
    }