十五子问题 （Mooc 里的例子， VS 2017，运行只有Button1,点击后显示错误）

Question

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 WindowsFormsApp8
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
const int N = 4; //按钮的行、列数
        Button[,] buttons = new Button[N, N]; //按钮的数组

    private void Form1_Load(object sender, EventArgs e)
    {

            //产生所有按钮
            GenerateAllButtons();
}
private void button1_Click(object sender, EventArgs e)
{
//打乱顺序
Shuffle();

    }
    //打乱顺序

        void Shuffle()
{
            //多次随机交换两个按钮
            Random rnd = new Random();
for (int i = 0; i < 100; i++)
{
int a = rnd.Next(N);
int b = rnd.Next(N);
int c = rnd.Next(N);
int d = rnd.Next(N);
Swap(buttons[a, b], buttons[c, d]);
}
}

    //生成所有的按钮
    void GenerateAllButtons()
    {
        int x0 = 100, y0 = 10, w = 45, d = 50;
        for (int r = 0; r < N; r++)
            for (int c = 0; c < N; c++)
            {
                int num = r * N + c;
                Button btn = new Button();
                btn.Text = (num + 1).ToString();
                btn.Top = y0 + r * d;
                btn.Left = x0 + c * d;
                btn.Width = w;
                btn.Height = w;
                btn.Visible = true;
                btn.Tag = r * N + c; //这个数据用来表示它所在行列位置

                //注册事件
                btn.Click += new EventHandler(btn_Click);

                buttons[r, c] = btn; //放到数组中

                    this.Controls.Add(btn); //加到界面上
                }
buttons[N - 1, N - 1].Visible = false; //最后一个不可见
        }

    //交换两个按钮
    void Swap(Button btna, Button btnb)
    {
        string t = btna.Text;
        btna.Text = btnb.Text;
        btnb.Text = t;

        bool v = btna.Visible;
        btna.Visible = btnb.Visible;
        btnb.Visible = v;
    }

    //按钮点击事件处理
    void btn_Click(object sender, EventArgs e)
    {
        Button btn = sender as Button; //当前点中的按钮

            Button blank = FindHiddenButton(); //空白按钮

        //判断是否与空白块相邻，如果是，则交换
        if (IsNeighbor(btn, blank))
        {
            Swap(btn, blank);
            blank.Focus();
        }

        //判断是否完成了
        if (ResultIsOk())
        {
            MessageBox.Show("ok");
        }
    }

    //查找要隐藏的按钮
    Button FindHiddenButton()
    {
        for (int r = 0; r < N; r++)
            for (int c = 0; c < N; c++)
            {
                if (!buttons[r, c].Visible)
                {
                    return buttons[r, c];
                }
            }
        return null;
    }

    //判断是否相邻
    bool IsNeighbor(Button btnA, Button btnB)
    {
        int a = (int)btnA.Tag; //Tag中记录是行列位置

            int b = (int)btnB.Tag;
int r1 = a / N, c1 = a % N;
int r2 = b / N, c2 = b % N;

        if (r1 == r2 && (c1 == c2 - 1 || c1 == c2 + 1) //左右相邻
                        || c1 == c2 && (r1 == r2 - 1 || r1 == r2 + 1))
            return true;
        return false;
    }

    //检查是否完成
    bool ResultIsOk()
    {
        for (int r = 0; r < N; r++)
            for (int c = 0; c < N; c++)
            {
                if (buttons[r, c].Text != (r * N + c + 1).ToString())
                {
                    return false;
                }
            }
        return true;
    }

}

}