用url访问webservice出错，

using System;
using System.Collections.Generic;
using System.Web;
using System.Web.Services;

namespace WebService1
{
    /// <summary>
    /// Service1 的摘要说明
    /// </summary>
    [WebService(Namespace = "http://tempuri.org/")]
    [WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
    [System.ComponentModel.ToolboxItem(false)]
    public class Service1 : System.Web.Services.WebService
    {

        [WebMethod]
        public string HelloWorld()
        {
            return "Hello World";
        }

        [WebMethod]
        public string Test(string a)
        {
            return a;
        }
    }
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Net;
using System.IO;

public partial class Default12 : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        string url = "http://localhost:9109/Service1.asmx/Test?a=123";

        HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
        request.Method = "GET";
        StreamReader sr = new StreamReader(request.GetResponse().GetResponseStream());
        string ret = sr.ReadToEnd();

        Response.Write(ret);
        Response.End();

    }
}