c＃为什么2个骰子总是相同的数字

Question

尝试获得2个骰子以滚动2个不同的数字，但无论我尝试什么，2个数字总是相同的。我无法解决为什么即使在完成所有这些之后，2个数字总是相同的，例如＆＃34; 6和6＆＃34; ＆＃34; 2和2＆＃34;。

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace WindowsFormsApplication7
{
    public partial class Form1 : Form
    {

        String[] rolldie()
        {
            int a = 0;
            int b = 0;
            string first = "";
            string second = "";
            for (int i=0; i<20; i++)
            {

                Random rnd = new Random();
                int dice = rnd.Next(1, 7);
                if (i == 0) { a = dice; }
                if (i == 10) { b = dice; }
                first = a.ToString();
                second = b.ToString();

            }
            string[] diceword = {first,second};
            return diceword;

        }

        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            textBox1.Text = rolldie()[0];
            textBox2.Text = rolldie()[1];

        }
    }
}