public static bool CreateCookie(string strCookieName, string strCookieValue, string strDomain, bool blURLEncode)
        {
            if (blURLEncode)
            {
                strCookieValue = System.Web.HttpContext.Current.Server.UrlEncode(strCookieValue);
            }
            HttpCookie objCookie = new HttpCookie(strCookieName, strCookieValue);
            objCookie.Domain = strDomain; //设置Cookie的域名
            System.Web.HttpContext.Current.Response.Cookies.Add(objCookie);
            return true;
        }

url:http://greatverve.cnblogs.com/archive/2011/07/05/asp-net-cookie-domain.html

Cookie有三个属性需要注意一下：
1. Domain 域
2. Path       路径
3. Expires 过期时间

跨域操作需要设置域属性:
Response.Cookies("MyCookie").Domain = "cnblogs.com"; (这里指的是泛域名)
这样在其它二级域名下就都可以访问到了， ASP 和 ASP.NET 测试通过

虚拟目录下访问：
我在ASP端做了下测试，.NET的没试, 如果不指定Path属性， 不同虚拟目录下Cookie无法共享
将Response.Cookies("MyCookie").Path = "/" 就可以了

总的写法:
Response.Cookies("MyCookie").Domain = "cnblogs.com";
Response.Cookies("MyCookie").Path = "/"
Response.Cookies("MyCookie").Expires = Now + 365;
Response.Cookies("MyCookie")("Test") = "test";

.NET 清除Cookie
HttpCookie cookie = System.Web.HttpContext.Current.Request.Cookies[cookiename];
if (cookie != null)
{
                cookie.Values.Clear();
                SetUserCookieExpireTime(cookiename, -1);
                cookie.Domain = _domain;
                System.Web.HttpContext.Current.Response.Cookies.Set(cookie);
}
public static void SetUserCookieExpireTime(string key, int days)
{
            System.Web.HttpContext.Current.Response.Cookies[key].Domain = _domain;
            System.Web.HttpContext.Current.Response.Cookies[key].Path = _cookiepath;
            System.Web.HttpContext.Current.Response.Cookies[key].Expires = DateTime.Now.AddDays(days);
}
.NET 添加/更新Cookie
public static void AddUserCookies(string key,string value, string cookiename, string domain)
{
            HttpCookie cookie = System.Web.HttpContext.Current.Request.Cookies[cookiename];
            if (cookie == null)
            {
                cookie = new HttpCookie(cookiename);
                cookie.Domain = domain;
                cookie.Path = _cookiepath;

cookie.Values.Add(key, value);
                HttpContext.Current.Response.AppendCookie(cookie);
            }
            else
            {
                if (System.Web.HttpContext.Current.Request.Cookies[cookiename].Values[key] != null)
                {
                    cookie.Values.Set(key, value);
                }
                else
                {
                    cookie.Domain = domain;
                    cookie.Path = _cookiepath;

cookie.Values.Add(key, value);
                    HttpContext.Current.Response.AppendCookie(cookie);
                }
            }
}

这种写法实现cookie跨域跨目录