C#编程，四则运算计算器，我编的个第一位可以输入0的，怎么让第一位输0时为空，还有除数不能为零这些问题

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

namespace 四则运算计算器
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        int flag;
        int num1;
        int results;

        private void num1_Click(object sender, EventArgs e)
        {
            Button a = (Button)sender;
            textBox1.Text = textBox1.Text + a.Text;
        }

        private void operator_Click(object sender, EventArgs e)
        {
            Button b = (Button)sender;
            if (b.Text == "+")
            {
                num1 = int.Parse(textBox1.Text);
                textBox1.Text = "";
                flag = 0;
            }
            if (b.Text == "-")
            {
                num1 = int.Parse(textBox1.Text);
                textBox1.Text = "";
                flag = 1;
            }
            if (b.Text == "*")
            {
                num1 = int.Parse(textBox1.Text);
                textBox1.Text = "";
                flag = 2;
            }
            if (b.Text == "/")
            {
                num1 = int.Parse(textBox1.Text);
                textBox1.Text = "";
                flag = 3;
            }
            if (b.Text == "C")
            {
                num1 = int.Parse(textBox1.Text);
                textBox1.Text = "";
                textBox1.Focus();
            }
            if (b.Text == "=")
            {
                if (flag == 0)
                {
                    results = num1 + int.Parse(textBox1.Text);
                }
                if (flag == 1)
                {
                    results = num1 - int.Parse(textBox1.Text);
                }
                if (flag == 2)
                {
                    results = num1 * int.Parse(textBox1.Text);
                }
                if (flag == 3)
                {
                    results = num1 / int.Parse(textBox1.Text);
                }
                textBox1.Text = results.ToString();
            }
        }

    }
}

追问：那怎么让第一位不输入0的呢不要这样的，第一位只能1-9，最好把代码给我一下哈

答案：
在获得除数时对它进行判断 if(除数 == 0 )  终止操作
这个我不会学C++编程！

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