Makhanov_Calc_2/Form1.cs

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 Makhanov_Calc_2
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        float a, b;
        int oper;
        bool znak = true;

        private void button4_Click(object sender, EventArgs e)
        {
            if (znak == true)
            {
                textBox1.Text = "-" + textBox1.Text;
                znak = false;
            }
            else if (znak == false)
            {
                textBox1.Text = textBox1.Text.Replace("-", "");
                znak = true;
            }
        }

        private void button1_Click(object sender, EventArgs e)
        {
            textBox1.Text = "";
            label1.Text = "";
        }

        private void button2_Click(object sender, EventArgs e)
        {
            int lenght = textBox1.Text.Length - 1;
            string text = textBox1.Text;
            textBox1.Clear();
            for (int i = 0; i < lenght; i++)
            {
                textBox1.Text = textBox1.Text + text[i];
            }
        }

        private void button3_Click(object sender, EventArgs e)
        {
            a = float.Parse(textBox1.Text);
            textBox1.Clear();
            oper = 1;
            label1.Text = a.ToString() + "+";
            znak = true;
        }

        private void button20_Click(object sender, EventArgs e)
        {
            textBox1.Text = textBox1.Text + 0;
        }

        private void button16_Click(object sender, EventArgs e)
        {
            textBox1.Text = textBox1.Text + 1;
        }

        private void button15_Click(object sender, EventArgs e)
        {
            textBox1.Text = textBox1.Text + 2;
        }

        private void button14_Click(object sender, EventArgs e)
        {
            textBox1.Text = textBox1.Text + 3;
        }

        private void button12_Click(object sender, EventArgs e)
        {
            textBox1.Text = textBox1.Text + 4;
        }

        private void button11_Click(object sender, EventArgs e)
        {
            textBox1.Text = textBox1.Text + 5;
        }

        private void button10_Click(object sender, EventArgs e)
        {
            textBox1.Text = textBox1.Text + 6;
        }

        private void button8_Click(object sender, EventArgs e)
        {
            textBox1.Text = textBox1.Text + 7;
        }

        private void button7_Click(object sender, EventArgs e)
        {
            textBox1.Text = textBox1.Text + 8;
        }

        private void button6_Click(object sender, EventArgs e)
        {
            textBox1.Text = textBox1.Text + 9;
        }

        private void button5_Click(object sender, EventArgs e)
        {
            a = float.Parse(textBox1.Text);
            textBox1.Clear();
            oper = 2;
            label1.Text = a.ToString() + "-";
            znak = true;
        }

        private void button9_Click(object sender, EventArgs e)
        {
            a = float.Parse(textBox1.Text);
            textBox1.Clear();
            oper = 3;
            label1.Text = a.ToString() + "*";
            znak = true;
        }

        private void button13_Click(object sender, EventArgs e)
        {
            a = float.Parse(textBox1.Text);
            textBox1.Clear();
            oper = 4;
            label1.Text = a.ToString() + "/";
            znak = true;
        }

        private void button19_Click(object sender, EventArgs e)
        {
            textBox1.Text = textBox1.Text + ",";
        }

        private void button18_Click(object sender, EventArgs e)
        {
            calculate();
            label1.Text = "";
        }

        private void calculate()
        {
            switch (oper)
            {
                case 1:
                    b = a + float.Parse(textBox1.Text);
                    textBox1.Text = b.ToString();
                    break;
                case 2:
                    b = a - float.Parse(textBox1.Text);
                    textBox1.Text = b.ToString();
                    break;
                case 3:
                    b = a * float.Parse(textBox1.Text);
                    textBox1.Text = b.ToString();
                    break;
                case 4:

                    if (float.Parse(textBox1.Text) == 0)
                    { textBox1.Text = "Делить на 0 нельзя!"; }
                    else
                    {
                        b = a / float.Parse(textBox1.Text);
                        textBox1.Text = b.ToString();
                    }
                    break;
                default:
                    break;
            }

        }

        private void textBox1_TextChanged(object sender, EventArgs e)
        {

        }

        private void Form1_Load(object sender, EventArgs e)
        {

        }
    }
}