为什么会出现异常，要怎么解决这个问题？
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;

namespace WindowsApplication1
{
public partial class caishu : Form
{
public caishu()
{
InitializeComponent();
}
static int[] Num = new int[8];//猜数系列。
int i;
private void button1_Click(object sender, EventArgs e)
{
Random m = new Random();
for ( i = 0; i < Num.Length; i++)
{
Num[i] = m.Next(100);


}
foreach (int a in Num)
{

for (int x = 0; x < i; x++)
{
if (Num[i] == Num[x])
{
MessageBox.Show("运气很好！");
}
else
{
MessageBox.Show("运气一般！");
}
}
textBox1.Text += a.ToString() + ",";
}


}


}
}

[解决办法]
for (int x = 0; x < i; x++)//i是什么，改成Num.Length
{
if (Num[i] == Num[x])
{
[解决办法]

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

namespace WindowsApplicati……

[解决办法]
for ( i = 0; i < Num.Length; i++)

i是有作用域的，出了这个for循环就不好使了，没有了就