public class TestHandler : System.Web.IHttpHandler
    {
        public bool IsReusable
        {
            get { return false; }
        }

        public void ProcessRequest(System.Web.HttpContext context)
        {
            Page page = BuildManager.CreateInstanceFromVirtualPath("~/webform1.aspx", typeof(WebForm1) ) as Page; 
// WebForm1是你要加载页面对应的后台类
            page.ProcessRequest(context);

        }
    }

public partial class fanshe : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        IHttpHandler page =BuildManager.CreateInstanceFromVirtualPath("/ascx/test.ashx", typeof (IHttpHandler)) as IHttpHandler;
        page.ProcessRequest(HttpContext.Current);
    }
}