效果如圖：

程式碼：

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

namespace _1csz
{
    public partial class Form1 : Form
    {
        int x;///定義的是一個全域性變數
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)///產生一個隨機數
        {
            Random rd = new Random();
            x = rd.Next(100);
        }

        private void button2_Click(object sender, EventArgs e)///顯示正確答案
        {
            label4.Visible = true;
            label4.Text = x.ToString();
        }

        private void button3_Click(object sender, EventArgs e)///退出鍵
        {
            Application.Exit();
        }

        private void Form1_Load(object sender, EventArgs e)///窗體初始化
        {
            label3.Visible = false;
            label4.Visible = false;
        }

        private void textBox1_KeyDown(object sender, KeyEventArgs e)///KeyDown事件：當焦點在文字框時按下任何鍵都觸發該事件
        {
            if (e.KeyCode==Keys.Enter)///KeyCode屬性獲取KeyUp和KeyDown事件的鍵盤程式碼，其值用Keys列舉成員名
            {
                if (x==int.Parse(textBox1.Text))
                {
                    label3.Visible = true;
                    label3.Text = "猜對了，你真棒！";
                }
                else if (int.Parse(textBox1.Text) > x)
                {
                    label3.Visible = true;
                    label3.Text = "真是，猜大了！";
                }
                else
                {
                    label3.Visible = true;
                    label3.Text = "真是，猜小了！";
                }
            }
        }

    }
}