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);

    }

}