How to make simple Calculator in c#

In this blog we will learn that how to make a basic calculator check the source code:
 
Basically in normal calculators u can use only buttons to execute the calculator in .net application but here is a liitle change in the scenario i added a hotkeys in this calculator (Like '+' to add ,'/' to divide) after press the numlock on button on calculator u can use numpad keys to execute this calculator so use it and enjoy the difference
 
source code:

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
WindowsFormsApplication33
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        string fun, no;
        private void button2_Click(object sender, EventArgs e)
        {
            textBox1.Text += button2.Text;
        }

        private void button1_Click(object sender, EventArgs e)
        {
            textBox1.Text += button1.Text;
        }

        private void button3_Click(object sender, EventArgs e)
        {
            if (textBox1.Text == string.Empty)
            {
                errorProvider1.SetError(textBox1, "Must enter the value to remove");
            }
            else
            {
                errorProvider1.Clear();
                no = textBox1.Text;
                int no1 = no.Length;
                textBox1.Text = (no.Substring(0, no1 - 1));
            }
        }

        private void button4_Click(object sender, EventArgs e)
        {
            textBox1.Text += button4.Text;
        }

        private void button5_Click(object sender, EventArgs e)
        {
            textBox1.Text += button5.Text;
        }

        private void button6_Click(object sender, EventArgs e)
        {
            textBox1.Text += button6.Text;
        }

        private void button7_Click(object sender, EventArgs e)
        {
            textBox1.Text += button7.Text;
        }

        private void button8_Click(object sender, EventArgs e)
        {
            textBox1.Text += button8.Text;
        }

        private void button9_Click(object sender, EventArgs e)
        {
            textBox1.Text += button9.Text;
        }

        private void button10_Click(object sender, EventArgs e)
        {
            textBox1.Text += button10.Text;
        }

        private void button11_Click(object sender, EventArgs e)
        {
            textBox1.Text += button11.Text;
        }

        private void button12_Click(object sender, EventArgs e)
        {
            textBox1.Text += button12.Text;
        }

        private void textBox1_TextChanged(object sender, EventArgs e)
        {
            errorProvider1.Clear();
            if (textBox1.Text == "+" || textBox1.Text == "-" || textBox1.Text == "*" ||
textBox1.Text == "/")
            {
                textBox1.Text = "";
            }
        }

        private void Form1_Load(object sender, EventArgs e)
        {

        }
        public void funcal()
        {
            if (textBox1.Text == "")
            {
                errorProvider1.SetError(textBox1, "Must enter the value to remove");
            }
            else
            {
                if (fun == "+")
                {
                    textBox1.Text = Convert.ToString(Convert.ToDecimal(no) +
Convert.ToDecimal(textBox1.Text));
                }
                else if (fun == "-")
                {
                    textBox1.Text = Convert.ToString(Convert.ToDecimal(no) -
Convert.ToDecimal(textBox1.Text));
                }
                else if (fun == "*")
                {
                    textBox1.Text = Convert.ToString(Convert.ToDecimal(no) *
Convert.ToDecimal(textBox1.Text));
                }
                else if (fun == "/")
                {
                    textBox1.Text = Convert.ToString(Convert.ToDecimal(no) /
Convert.ToDecimal(textBox1.Text));
                }
            }
        }

        private void button16_Click(object sender, EventArgs e)
        {
            no = textBox1.Text;
            textBox1.Text = "";
            fun = "+";
        }

        private void button15_Click(object sender, EventArgs e)
        {
            no = textBox1.Text;
            textBox1.Text = "";
            fun = "-";
        }

        private void button14_Click(object sender, EventArgs e)
        {
            no = textBox1.Text;
            textBox1.Text = "";
            fun = "*";
        }

        private void button13_Click(object sender, EventArgs e)
        {
            no = textBox1.Text;
            textBox1.Text = "";
            fun = "/";
        }

        private void button17_Click(object sender, EventArgs e)
        {
            funcal();
        }

        private void button18_Click(object sender, EventArgs e)
        {
            textBox1.Text = "";
            no = "";
            fun = "";
        }

        private void button16_KeyDown(object sender, KeyEventArgs e)
        {

        }

        private void textBox1_KeyDown(object sender, KeyEventArgs e)
        {

        }

        private void checkBox1_CheckedChanged(object sender, EventArgs e)
        {

        }

        private void button16_KeyDown_1(object sender, KeyEventArgs e)
        {

        }


        private void checkBox1_CheckedChanged_1(object sender, EventArgs e)
        {
            if (checkBox1.Checked)
            {
                textBox1.ReadOnly = false;
                checkBox1.Text = "Num Lock Off";
                textBox1.Select();
            }
            else
            {
                textBox1.ReadOnly = true;
                checkBox1.Text = "Num Lock On";
                button17.Select();
            }
        }

        private void textBox1_KeyDown_1(object sender, KeyEventArgs e)
        {
            if (e.KeyCode == Keys.Add)
            {
                no = textBox1.Text;
                textBox1.Text = "";
                fun = "+";
            }
            else if (e.KeyCode == Keys.Subtract)
            {
                no = textBox1.Text;
                textBox1.Text = "";
                fun = "-";
            }
            else if (e.KeyCode == Keys.Multiply)
            {
                no = textBox1.Text;
                textBox1.Text = "";
                fun = "*";
            }
            else if (e.KeyCode == Keys.Divide)
            {
                no = textBox1.Text;
                textBox1.Text = "";
                fun = "/";
            }
            else if (e.KeyCode == Keys.Enter)
            {
                funcal();
            }
            else if (e.KeyCode == Keys.Escape)
            {
                textBox1.Text = "";
                no = "";
                fun = "";
            }
        }

        private void button20_Click(object sender, EventArgs e)
        {
            textBox1.Text = Convert.ToString(System.Math.Sqrt(Convert.ToDoubl
(textBox1.Text)));
        }

        private void button19_Click(object sender, EventArgs e)
        {
            textBox1.Text = Convert.ToString(Convert.ToDecimal(textBox1.Text) *
Convert.ToDecimal(textBox1.Text));
        }

        private void button21_Click(object sender, EventArgs e)
        {
            textBox1.Text = Convert.ToString(1 / Convert.ToDecimal(textBox1.Text));
        }
 
        private void button22_Click(object sender, EventArgs e)
        {
 
        }
    }
}