using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApp
{
    class Program
    {
        static string hashToPostString(Hashtable ht) {
            string str = "";
            foreach (DictionaryEntry de in ht) {
                str += de.Key + "=" + de.Value + "&";
            }
            return str.Substring(0, str.Length - 1);
        }

        static void Main(string[] args)
        {
            Hashtable ht = new Hashtable();
            ht.Add("username", "13713332652");
            ht.Add("password", "202063sb");
            ht.Add("geetest_challenge", "3c2f03027eb7cac324a7cf67f148441d");
            ht.Add("geetest_validate", "21fa24dfd285b955776fd349c5bc5834");
            ht.Add("geetest_seccode", "21fa24dfd285b955776fd349c5bc5834|jordan");            

            string str = hashToPostString(ht);
            Console.Write(str);
            Console.ReadLine();
        }
    }
}

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApp
{
    class Program
    {
        static string DictionaryToPostString(Dictionary<string, string> ht) {
            string str = "";
            foreach (KeyValuePair<string, string> de in ht) {
                str += de.Key + "=" + de.Value + "&";
            }
            return str.Substring(0, str.Length - 1);
        }

        static void Main(string[] args)
        {

            //定义字典  
            Dictionary<string, string> d = new Dictionary<string, string>();
            d.Add("gt", "geetest.gt");
            d.Add("challenge", "geetest.challenge");
            d.Add("model", "3");
            d.Add("referer", "http://www.228.com.cn/auth/login");
            d.Add("return", "json");
            d.Add("user", "dragon8jiyan");
            d.Add("pass", "202063");

            string str = DictionaryToPostString(d);
            Console.Write(str);
            Console.ReadLine();
        }
    }
}