WebRequest request = asyncResult.AsyncState as WebRequest;
            Stream stream = request.EndGetRequestStream(asyncResult);
            StreamWriter sw = new StreamWriter(stream);
           // sw.Write(string.Format("No={0}",lb_Books.SelectedIndex.ToString()));//这样写直接报跨线程访问无效
            sw.Write(string.Format("No={0}", new GetValue(GetVal)));//这种自定义委托实现
            sw.Flush();
            sw.Close();
            sw.Dispose();
            stream.Close();
            stream.Dispose();
//以下是委托部分：
private delegate string GetValue();
        private string GetVal()
        {
            return lb_Books.SelectedIndex.ToString();
        }

//ashx中，在获取提交的No时，得到时数据：
context.Request.Form["No"]
结果："SilverLightControl.SilverlightControl GetValue"本来应该获取的是lb_Books选中的索引值
context.Response.Write(context.Request.Form["No"]);