一个方法的同步与异步的问题？急需返回值

Question

 /// <summary>
    /// 传经纬度得到地址
    /// </summary>
    /// <param name="longitude">经度</param>
    /// <param name="latitude">纬度</param>
    /// <returns>详细地址</returns>
    public string GetAddressByGPS(double longitude, double latitude, object vehicleInfo)
    {
      int iLng = (int)(longitude * 100000);
      int iLat = (int)(latitude * 100000);

      WebRequest request = WebRequest.Create(Config.BMSSettings.LingTuGPS + iLng.ToString()
              + "," + iLat.ToString() + "&t=" + m_rand.Next());
      request.Method = "get";
      request.ContentType = "application/x-www-form-urlencoded";
      request.Timeout = 5000;//设置超时
      string strAddress = null;
      IAsyncResult Ias = request.BeginGetResponse((ar1) =>
        {
          try
          {
            WebResponse response = request.EndGetResponse(ar1);
            using (StreamReader sr = new StreamReader(response.GetResponseStream(), Encoding.GetEncoding(936)))
            {
              strAddress = sr.ReadToEnd();
              XPathDocument xpathDoc = new XPathDocument(new StringReader(strAddress));
              XPathNavigator xpathNav = xpathDoc.CreateNavigator();
              XPathNavigator nodeCode = xpathNav.SelectSingleNode("//code");

              if (nodeCode.InnerXml == "0")
              {
                XPathNavigator nodeMsg = xpathNav.SelectSingleNode("//msg");
               【strAddress = nodeMsg.InnerXml;】               
              }
              else
              {
                strAddress = "无法获取地址";

              }
            }
            response.Close();
            LogHelper.Writeln("In GetAddressByGPS()");

          }
          catch
          {
            strAddress = "无法获取地址";
            LogHelper.Writeln("In GetAddressByGPS() In Error");

          }
        }, null);
       return strAddress;
}

执行完到标注的时候，无法执行后面的return，也就调用这个方法得不到strAddress 值，请求是异步的，主方法是同步的，但是我要获取这个返回值，真的不知道怎么办了？