请大家帮忙优化下代码!该如何解决

请大家帮忙优化下代码!
以前一直是做WEB开发,ASP.NET,最近公司要求做一个能生成5万条互不相同的字符串的功能.
用VS2005创建windows程序,windows程序写好后测试能生成5万条.但是效率很低,花了3分钟时间,CPU使用率在98%-100%之间徘徊.
我的代码如下,想请各位帮忙给优化下,让代码跑起来快些.(初次写windows程序,有很多不懂,希望在****请教各位达人!)
Form1.cs代码:
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.IO;
using System.Text;

namespace createtxt
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}

private void button1_Click(object sender, EventArgs e)
{
string code = "";
for (int i = 1; i <= 50000; i++)
{
code += rc(i) + "\r\n";
}
label1.Text="已生成五万条随机字符串";
string path = "D:/";
Encoding codee = Encoding.GetEncoding("UTF-8");
StreamWriter sw = new StreamWriter(path + "string.txt", false, codee);
sw.Write(code);
sw.Flush();
sw.Close();
label1.Text = "生成文件成功,文件名为:string.txt";
}

public static string rc(int a)
{
char[] arrchar = new char[]{
'0','1','2','3','4','5','6','7','8','9',
'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'
};
StringBuilder num = new StringBuilder();
Random rnd = new Random(a);
for (int i = 1; i <= 4; i++)
{
//num2.Append(arrchar[rnd.Next(0,10)].ToString());
//num.Append(arrchar[rnd.Next(0, 10)].ToString());
num.Append(arrchar[rnd.Next(11, arrchar.Length)].ToString());
}
for (int j = 1; j <= 8; j++)
{
num.Append(arrchar[rnd.Next(0, 10)].ToString());
}

return num.ToString();
}
}
}

------解决方案--------------------
在我机器上耗时125毫秒

C# code

private const int CODE_COUNT = 50000; // 记录数
private byte[] codeBuffer = new byte[14 * CODE_COUNT];

private Random random = new Random(); 
public byte[] RandomCode()
{
    byte[] vReturn = new byte[14];
    vReturn[12] = 13;
    vReturn[13] = 10;
    for (int i = 0; i < 4; i++)
        vReturn[i] = (byte)('A' + random.Next(26));
    for (int i = 4; i < 12; i++)
        vReturn[i] = (byte)('0' + random.Next(10));
    return vReturn;
}

private Hashtable vHashtable = new Hashtable();

private void button1_Click(object sender, EventArgs e)
{
    long vTickCount = Environment.TickCount;
    byte[] code = RandomCode();
    for (int i = 0; i < CODE_COUNT; )
    {
        code = RandomCode();
        if (!vHashtable.Contains(code)) // 避免重复
        {
            vHashtable.Add(code, null);
            Array.Copy(code, 0, codeBuffer, i * 14, code.Length);
            i++;
        }
    }
    FileStream vFileStream = new FileStream(@"c:\temp\temp.txt", 
        FileMode.Create, FileAccess.Write);
    int vPos = 0;
    for (; vPos + 0x1000 < codeBuffer.Length; vPos += 0x1000)
    {
        vFileStream.Write(codeBuffer, vPos, 0x1000); // 减少文件写入的次数，采用每4096字节写一次。
    }
    vFileStream.Write(codeBuffer, vPos, codeBuffer.Length - vPos);
    vFileStream.Close();
    Console.WriteLine("耗时：{0}毫秒", Environment.TickCount - vTickCount);
}

请大家帮忙优化下代码!该如何解决

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