求帮忙看看，怎么一直报错

using System;
using System.Collections.Generic;
using System.Web;
using System.Text;
using System.Data;
namespace DTcms.Web.tools
{
    /// <summary>
    /// city_ajax 的摘要说明
    /// </summary>
    public class city_ajax : IHttpHandler
    {        public void ProcessRequest(HttpContext context)
        {
            if (!string.IsNullOrEmpty(context.Request.QueryString["type"]))
            {
                switch (context.Request.QueryString["type"])
                {
                    case "GetCityInfo":
                        GetCityInfo(context);
                        break;
                    default:
                        break;
                }
            }
        }        public void GetCityInfo(HttpContext context)
        {
            if (!string.IsNullOrEmpty(context.Request.QueryString["proId"])
                && !string.IsNullOrEmpty(context.Request.QueryString["level"]))
            {
                string controlName = "";
                switch (context.Request.QueryString["level"])
                {
                    case "1":
                        controlName = "ddlCity";
                        break;
                    default:
                        return;
                }
                BLL.dt_city bll = new BLL.dt_city();
                DataSet ds = bll.CityGetList(int.Parse(context.Request.QueryString["proId"]));                DataTable dt = ds.Tables[0];
                StringBuilder sb = new StringBuilder();
                sb.Append(String.Format("<select name=\"{0}\" ", controlName));
                sb.Append("<option value=\"0\">未选择</option>");
                foreach (DataRow row in dt.Rows)
                {                    sb.Append("<option value=\"");
                    sb.Append(row["id"].ToString());
                    sb.Append("\">");
                    sb.Append(row["name"].ToString());
                    sb.Append("</option>");
                }
                sb.Append("</select>");                context.Response.Write(sb.ToString());
            }
        }        public bool IsReusable
        {
            get
            {
                return false;
            }
        }
    }
}