asp.net 如何跳出一个选择性的(是或者否)对话框

我写了以下代码不起作用,不知道为什么
public partial class _Default : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
HttpPostedFile p;
if (Request.QueryString["type"] == "true")
{
ViewState["a"] = '5';
}
if (!Page.IsPostBack)
{
System.Data.DataSet ds = new System.Data.DataSet();
System.Data.DataTable dataTable1 = new System.Data.DataTable("BlogUser");
System.Data.DataRow dr;
dataTable1.Columns.Add(new System.Data.DataColumn("UserId", typeof(System.Int32)));
dataTable1.Columns.Add(new System.Data.DataColumn("UserName", typeof(System.String)));

for (int i = 0; i < 8; i++)
{
dr = dataTable1.NewRow();
dr[0] = i;
dr[1] = "【孟子E章】" + i.ToString();
dataTable1.Rows.Add(dr);
}
ListView1.DataSource = dataTable1;
ListView1.DataBind();
}
}

protected void Button1_Click(object sender, EventArgs e)
{
String UserId = "";
for (int i = 0; i < ListView1.Items.Count; i++)
{
CheckBox chktemp = ListView1.Items[i].FindControl("CheckBox1") as CheckBox;
if (chktemp != null && chktemp.Checked == true)
{
UserId += ListView1.DataKeys[i].Value + ",";

}
if (i == 4)
{
ViewState["a"] = '1';
Response.Write("<script>location.href='Default.aspx?type='+confirm('该学生此学科成绩已存在,是否覆盖此成绩？');</script");
if (Request.QueryString["type"] == "true")
{
Response.Write(ViewState["a"].ToString());
}
}
}
Page.ClientScript.RegisterStartupScript(Page.GetType(), "x", "alert('有选中 id=" + UserId + "')", true);
}


}