//取得cookie    
function getCookie(name) {    
 var nameEQ = name + "=";    
 var ca = document.cookie.split(';');    //把cookie分割成组    
 for(var i=0;i < ca.length;i++) {    
 var c = ca[i];                      //取得字符串    
 while (c.charAt(0)==' ') {          //判断一下字符串有没有前导空格    
 c = c.substring(1,c.length);      //有的话，从第二位开始取    
 }    
 if (c.indexOf(nameEQ) == 0) {       //如果含有我们要的name    
 return unescape(c.substring(nameEQ.length,c.length));    //解码并截取我们要值    
 }    
 }    
 return false;    
}    
    
//清除cookie    
function clearCookie(name) {    
 setCookie(name, "", -1);    
}    
    
//设置cookie    
function setCookie(name, value, seconds) {    
 seconds = seconds || 0;   //seconds有值就直接赋值，没有为0，这个根php不一样。    
 var expires = "";    
 if (seconds != 0 ) {      //设置cookie生存时间    
 var date = new Date();    
 date.setTime(date.getTime()+(seconds*1000));    
 expires = "; expires="+date.toGMTString();    
 }    
 document.cookie = name+"="+escape(value)+expires+"; path=/";   //转码并赋值    
}    

调用一下上面方法：  

setCookie("test","tank",1800);         //设置cookie的值，生存时间半个小时    
alert(getCookie('test'));                     //取得cookie的值，显示tank    
clearCookie("test");                           //删除cookie的值    
alert(getCookie('test'));                     //test对应的cookie值为空，显示为false.就是getCookie最后返的false值。    

using System;
using System.Data;
using System.Configuration;
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;

 
public class Cookie
{
    /// <summary>
    /// Cookies赋值
    /// </summary>
    /// <param name="strName">主键</param>
    /// <param name="strValue">键值</param>
    /// <param name="strDay">有效天数</param>
    /// <returns></returns>
    public bool setCookie(string strName, string strValue, int strDay)
    {
        try
        {
            HttpCookie Cookie = new HttpCookie(strName);
            //Cookie.Domain = ".xxx.com";//当要跨域名访问的时候,给cookie指定域名即可,格式为.xxx.com
            Cookie.Expires = DateTime.Now.AddDays(strDay);
            Cookie.Value = strValue;
            System.Web.HttpContext.Current.Response.Cookies.Add(Cookie);
            return true;
        }
        catch
        {
            return false;
        }
    }

    /// <summary>
    /// 读取Cookies
    /// </summary>
    /// <param name="strName">主键</param>
    /// <returns></returns>
 
    public string getCookie(string strName)
    {
        HttpCookie Cookie = System.Web.HttpContext.Current.Request.Cookies[strName];
        if (Cookie != null)
        {
            return Cookie.Value.ToString();
        }
        else
        {
            return null;
        }
    }

    /// <summary>
    /// 删除Cookies
    /// </summary>
    /// <param name="strName">主键</param>
    /// <returns></returns>
    public bool delCookie(string strName)
    {
        try
        {
            HttpCookie Cookie = new HttpCookie(strName);
            //Cookie.Domain = ".xxx.com";//当要跨域名访问的时候,给cookie指定域名即可,格式为.xxx.com
            Cookie.Expires = DateTime.Now.AddDays(-1);
            System.Web.HttpContext.Current.Response.Cookies.Add(Cookie);
            return true;
        }
        catch
        {
            return false;
        }
    }
}

Cookie Cookie = new Cookie();
Cookie.setCookie("name", "aaa",1);//赋值
Cookie.getCookie("name");//取值
Cookie.delCookie("name");//删除
注意:当Cookie存中文出现乱码,则在存放时给中文编码,如Cookie.setCookie("name", Server.UrlEncode("aaa"),1),读取时解码即可


另外：只要不给cookie设置过期时间,cookie在浏览器关闭的时候自动失效