//错误的写法
public void ProcessRequest(HttpContext context)
{
    SPSecurity.RunWithElevatedPrivileges(delegate
    {
       // 'SPContext.Current' null reference error
        using (var site = new SPSite(SPContext.Current.Site.ID))
        {
            using (var web = site.OpenWeb(SPContext.Current.Web.ID))
            {
               // codes goes here
            }
        }
    });

}

//正确的写法
public void ProcessRequest(HttpContext context)
{
    var curSite = SPContext.Current.Site;
    var curWeb = SPContext.Current.Web;
    SPSecurity.RunWithElevatedPrivileges(delegate
    {
        using (var site = new SPSite(curSite.ID))             {
            using (var web = site.OpenWeb(curWeb.ID))
            {
                // code goes here
            }
        }
    });

}