求助asp.net！ - .NET技术 / ASP.NET

刚刚学习asp.net，麻烦大家看看下面的代码错误，
  using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

public partial class Account_application : System.Web.UI.Page
{

  protected void Page_Load(object sender,EventArgs e)
  {
  if(!IsPostBack)
  this.Application["Data"]=System.DateTime.Now.ToString();
  }
  protected void btnSubmit_Click(object sender,EventArgs e)
  {
  StringBuilder sb = new StringBuilder();
  foreach (string str in this.Application.Allkeys)
  {
  sb.Append("键值:" + str);
  sb.Append("--对应的值为:" + Application[str]);
  sb.Append("<br/>");
  }
  this.lblInfo.Text = sb.ToString();
  }

}

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报什么错也不说下？？

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this.Application，不用this

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Application[str]==null?"":Application[str]

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Application不属于哪个页面，不需要this。

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请问错误在哪里？ 你怎么不把错误贴出来？

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sorry，发错误代码：
 error CS0246: The type or namespace name 'StringBuilder' could not be found (are you missing a using directive or an assembly reference?)
 error CS1061: 'System.Web