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

namespace 九九乘法表
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            this.textBox1.Visible = true;
            
            for (int i = 1; i <= 9; i++)
            {
                for (int j = 1; j <= 9; j++)
                {
                    if (i >= j)
                    {
                        Console.WriteLine(j + "*" + i + "=" + i * j);
                    }
                }
            }

        }
    }
}

private void button1_Click(object sender, EventArgs e)
        {
            this.textBox1.Visible = true;
            StringBuilder sb = new StringBuilder();
            for (int i = 1; i <= 9; i++)
            {
                for (int j = 1; j <= 9; j++)
                {
                    if (i >= j)
                    {
                        sb.Append(j + "*" + i + "=" + i*j + "\r\n");
                    }
                }
            }
            this.textBox1.Text = sb.ToString();

        }


我没怎么调格式，自己处理下吧。

在构造函数里加：
    this.textBox1.Multiline = true;

首先，你这个输出是应该在控制台应用程序里面；
其次，你的输出的逻辑有点问题，在内部for循环完成的时候可以输出也可以把所有的结果都保存在一个字符串里面最后输出；
下面是略加更改的程序：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace _99
{
    class Program
    {
        static void Main(string[] args)
        {
            string temp = "";
            for (int i = 1; i <= 9; i++)
            {
                for (int j = 1; j <= 9; j++)
                {
                    if (i >= j)
                    {
                        temp += i + "*" + j + "=" + i * j + "  ";
                    }
                } temp += "\n";
            }
            Console.WriteLine(temp);
            Console.ReadLine();
        }
    }
}

应该建一个控制台项目