ObjState obs = (ObjState)ar.AsyncState;
            WebRequest wr = obs.HWR;
            WebResponse hwresp = (WebResponse)wr.EndGetResponse(ar);//此处报错说：无法得到资源程序集

下面是objstate类，我实在看不出有什么错。

    public class ObjState
    {
        //此类为异步回调中对象状态类
        public WebRequest HWR;
        public long WebFileSize;

        public ObjState()
        {
            HWR = null;
        }
    }