JS代码Confirm确认和取消判断执行不同的事件

Default.aspx页面：
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
    <title>Are you Ready?</title>
</head>
<body>
    <form id="form1" runat="server">
        <asp:HiddenField ID="HiddenField1" runat="server" />
        <asp:button id="Button1" runat="server" Text="Are you ready?" OnClick="Button1_Click"></asp:button>
    </form>
</body>
</html>
Default.cs页面
using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Text;

public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        StringBuilder strbConfirmScript = new StringBuilder();

strbConfirmScript.Append("<script language='javascript' type='text/javascript'>\n");

        strbConfirmScript.Append("function showConfirm()\n");
        strbConfirmScript.Append("{\n var returnValue=confirm('Are you Sure?');\n");
        strbConfirmScript.Append(" if(returnValue!=undefined) \n document.getElementById('HiddenField1').value=returnValue; \n");
        strbConfirmScript.Append("}\n");
        strbConfirmScript.Append("</script>\n");

ClientScript.RegisterClientScriptBlock(this.GetType(), "AreYouConfirm", strbConfirmScript.ToString());

        Button1.Attributes.Add("onclick", "showConfirm()");
    }
    protected void Button1_Click(object sender, System.EventArgs e)
    {
        if (HiddenField1.Value == "true")
            ClientScript.RegisterStartupScript(this.GetType(), "script", "alert('A');", true);
        else
            ClientScript.RegisterStartupScript(this.GetType(), "script", "alert('B');", true);
    }
}