asp.net中viewState的应用

我们先看网上的一对问答     

private int x;

protected void Page_Load(object sender, EventArgs e)

{

     if (!IsPostBack)

     {

         x = 1;

     }

}

protected void Button1_Click(object sender, EventArgs e)

     x++;

     Response.Write(x.ToString());

这里每次调用都输出1，为什么不是递增？如果我想定义在当前页的变量如何定义。

这是因为x只是一个局部变量，在一次的网页请求，等到网页执行完毕的时候就会被回收，这时候x就已经不存在了，下次再访问的时候又会是一个新的x变量。

除了传统的Asp中的Session对象外，Asp.net提供了一个更好的ViewState对象。ViewState对象用来保存页面中的各种变量，甚至是对象。使用方法和HashTable类似，只要用变量名称做索引，如ViewState["Var"]，就可以用存取变量Var的值，而不管Var是普通变量，还是对象，甚至是内存中的一张DataTable，太方便了。为什么可以用ViewState而不能用static变量哪？原因就是服务器端会为每个连接到该页面的用户分别建立一个ViewState，所以ViewState相当于页面级的Session。这下我们可以放心地使用ViewState来存取需要暂存的变量和对象了。

典型应用：查询以后绑定

aspx关键代码

         <tr>

            <td align="center" style="padding-bottom: 20px; font-weight: bold; padding-top: 20px">

                所在院系<asp:DropDownList ID="drpCollege" runat="server" Height="22px" Width="140px">

                </asp:DropDownList>

                   学生姓名<asp:TextBox ID="txtName" runat="server"></asp:TextBox>

                    <asp:Button ID="btnQuery" runat="server" Text="查询"

                    onclick="btnQuery_Click" />

            </td>

        </tr>

        <tr>

            <td align="center" style="padding-bottom: 20px; padding-top: 20px">

                <br />

                <asp:GridView ID="GridView1" runat="server" DataKeyNames="Num" AllowSorting="true"

                    AutoGenerateColumns="false" CellPadding="5" GridLines="Both" BorderColor="Black"

                    Width="90%">

                </asp>

DAL关键代码：

        public IEnumerable<M_Student> ReadStuByCollegeAndName(String collnum, String name)

        {

            return from s in dc.M_Student

                   where

                       (!String.IsNullOrEmpty(collnum) ? s.CollegeNum.Equals(collnum) : true) &&

                       (!string.IsNullOrEmpty(name) ? s.Name.Contains(name) : true)

                   select s;

        } 

aspx.cs关键代码

 using System;

using System.Collections.Generic;

using System.Linq;

using System.Web;

using System.Web.UI;

using System.Web.UI.WebControls;

using OnlineTest.Datalinq;

namespace OnlineTest.Manager

    public partial class StudentInfoMaintain : BasePage

    {

        protected void Page_Load(object sender, EventArgs e)

            if (!IsPostBack)

            {

                Bind();

                .......................................

             }

        }

        public void BindGridView(String cnum,String name)

            IEnumerable<M_Student> models = BLL.M_StudentBLL.ReadStuByCollegeAndName(cnum, name);

            if (null != models && 0 != models.Count())

                // 起始条数

                int startRecord = AspNetPager1.PageSize * (AspNetPager1.CurrentPageIndex - 1);

                // 每页数目

                int maxRecords = AspNetPager1.PageSize;

                // 总数目

                this.AspNetPager1.RecordCount = models.Count();

                this.AspNetPager1.AlwaysShow = true;

                this.GridView1.DataSource = models.Skip(startRecord).Take(maxRecords);

                this.GridView1.DataBind();

            }

            else

                this.GridView1.DataSource = null;

        public void Bind()

            if (null == ViewState["num"])

                ViewState["num"] = "";

            if (null == ViewState["name"])

                ViewState["name"] = "";

            BindGridView(ViewState["num"].ToString(), ViewState["name"].ToString());

        // 分页事件

        protected void AspNetPager1_PageChanging(object src, Wuqi.Webdiyer.PageChangingEventArgs e)

            this.AspNetPager1.CurrentPageIndex = e.NewPageIndex;

            Bind();

        /// <summary>

        ///  查询

        /// </summary>

        /// <param name="sender"></param>

        /// <param name="e"></param>

        protected void btnQuery_Click(object sender, EventArgs e)

            if(!this.drpCollege.SelectedValue.Equals("0"))

                ViewState["num"] = this.drpCollege.SelectedValue;

            if (!String.IsNullOrEmpty(this.txtName.Text))

                ViewState["name"] = this.txtName.Text;

    }