WEBCLIENT,POST,参数,乱码

Question

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Net;
using System.Text.RegularExpressions;namespace Translaction
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }        private void button1_Click(object sender, EventArgs e)
        {
            this.Close();
        }        private void btn_translate_Click(object sender, EventArgs e)
        {
            if (!postData())
            {
                System.Windows.Forms.MessageBox.Show("发送错误");
            }
        }        //发送数据
        public bool postData()
        {
            string source = null;
            try
            {
                WebClient web = new WebClient();
                string url = "http://trans.godict.com/index.php";
                //string url="http://localhost/qq/test.asp";
                string langpair = "";
                source = txt_source.Text;
    
                switch (comb1.SelectedIndex)
                {
                    case 0:
                        langpair = "en_zh";
                        break;
                    case 1:
                        langpair = "zh_en";
                        break;
                    default:
                        langpair = "en_zh";
                        break;
                }
                string param = "from_content=" + source + "&langpair=" + langpair;
                web.Headers.Add("Content-Type", "application/x-www-form-urlencoded");
                byte[] postdata = Encoding.ASCII.GetBytes(param);                byte[] responseData = web.UploadData(url, "POST", postdata);
                string htmlString = Encoding.Default.GetString(postdata);                txt_result.Text = htmlString;
                //setTxt_result(htmlString);
                return true;
            }
            catch(Exception e)
            {
                txt_result.Text ="Error:"+ e.ToString();
                return false;
            }
        }             //设置结果
        private void setTxt_result(string source)
        {
            string a = "";
            Regex reg = new Regex("<textarea name=q (.+)?</textarea>");
            MatchCollection myMatch=reg.Matches(source);
            foreach (Match mm in myMatch)
            {
                a += mm+"\n";
            }
            Regex reg1 = new Regex(">(.+)?<");
            string rr = reg1.Match(a).ToString();
            txt_result.Text = rr.Substring(1, rr.Length - 2);
            
        }        private void Form1_Load(object sender, EventArgs e)
        {
            comb1.Items.Add("英文到中文");
            comb1.Items.Add("中文到英文");
            comb1.SelectedIndex = 0;
        }        private void comb1_SelectedIndexChanged(object sender, EventArgs e)
        {        }
    }
}
在WEB服务器端接收到的参数是乱码，请大家帮忙