Calculator In with a Flat Design C#

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
namespace Calculator
{
    public partial class Form1: Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        //vars
        string oper;
        decimal num1, num2, result = 0;
        private void Form1_Load(object sender, EventArgs e)
        {
            // txtDisplay.Text = "0";
        }
        //1
        private void btn1_Click(object sender, EventArgs e)
        {
            txtDisplay.Text += btn1.Text;
        }
        //2
        private void btn2_Click(object sender, EventArgs e)
        {
            txtDisplay.Text += btn2.Text;
        }
        //3
        private void btn3_Click(object sender, EventArgs e)
        {
            txtDisplay.Text += btn3.Text;
        }
        //4
        private void btn4_Click(object sender, EventArgs e)
        {
            txtDisplay.Text += btn4.Text;
        }
        //5
        private void btn5_Click(object sender, EventArgs e)
        {
            txtDisplay.Text += btn5.Text;
        }
        //6
        private void btn6_Click(object sender, EventArgs e)
        {
            txtDisplay.Text += btn6.Text;
        }
        //7
        private void btn7_Click(object sender, EventArgs e)
        {
            txtDisplay.Text += btn7.Text;
        }
        //8
        private void btn8_Click(object sender, EventArgs e)
        {
            txtDisplay.Text += btn8.Text;
        }
        //9
        private void btn9_Click(object sender, EventArgs e)
        {
            txtDisplay.Text += btn9.Text;
        }
        //0
        private void btn0_Click(object sender, EventArgs e)
        {
            txtDisplay.Text += btn0.Text;
        }
        //,
        private void btnComma_Click(object sender, EventArgs e)
        {
            txtDisplay.Text += btnComma.Text;
        }
        // +|-
        private void btnPlusOrMinus_Click(object sender, EventArgs e)
        {
            if (txtDisplay.Text.Contains("-"))
            {
                txtDisplay.Text = txtDisplay.Text.Remove(0, 1);
            }
            else
            {
                txtDisplay.Text = "-" + txtDisplay.Text;
            }
        }
        //Plus
        private void btnPlus_Click(object sender, EventArgs e)
        {
            num1 = decimal.Parse(txtDisplay.Text);
            oper = "+";
            txtDisplay.Text = string.Empty;
            textBox1.Text = num1.ToString() + " + ";
        }
        //SUB
        private void btnSub_Click(object sender, EventArgs e)
        {
            num1 = decimal.Parse(txtDisplay.Text);
            oper = "-";
            txtDisplay.Text = string.Empty;
            textBox1.Text = num1.ToString() + " - ";
        }
        //multiply
        private void btnTimes_Click(object sender, EventArgs e)
        {
            num1 = decimal.Parse(txtDisplay.Text);
            oper = "x";
            txtDisplay.Text = string.Empty;
            textBox1.Text = num1.ToString() + " x ";
        }
        //divide
        private void btnDivide_Click(object sender, EventArgs e)
        {
            num1 = decimal.Parse(txtDisplay.Text);
            oper = "/";
            txtDisplay.Text = string.Empty;
            textBox1.Text = num1.ToString() + " / ";
        }
        private void btnEqual_Click(object sender, EventArgs e)
        {
            num2 = decimal.Parse(txtDisplay.Text);
            textBox1.Text = string.Empty;
            switch (oper)
            {
                case "+":
                    result = num1 + num2;
                    txtDisplay.Text = result.ToString();
                    break;
                case "-":
                    result = num1 - num2;
                    txtDisplay.Text = result.ToString();
                    break;
                case "x":
                    result = num1 * num2;
                    txtDisplay.Text = result.ToString();
                    break;
                case "/":
                    if (num2 == 0)
                    {
                        MessageBox.Show(num1 + " cannot be divided by zero", "Error", MessageBoxButtons.OK, MessageBoxIcon.Information);
                        txtDisplay.Text = "0";
                    }
                    else
                    {
                        result = num1 / num2;
                        txtDisplay.Text = result.ToString();
                    }
                    break;
                case "%":
                    result = num1 % num2;
                    txtDisplay.Text = result.ToString();
                    break;
            }
        }
        //x^2
        private void btnXsquare_Click(object sender, EventArgs e)
        {
            result = decimal.Parse(txtDisplay.Text) * decimal.Parse(txtDisplay.Text);
            textBox1.Text = txtDisplay.Text + " ^ " + 2 + " ";
            txtDisplay.Text = result.ToString();
        }
        //%
        private void btnPercentage_Click(object sender, EventArgs e)
        {
            num1 = decimal.Parse(txtDisplay.Text);
            oper = "%";
            txtDisplay.Text = string.Empty;
            textBox1.Text = num1.ToString() + " % ";
        }
        private void btnErease_Click(object sender, EventArgs e)
        {
            string str1 = txtDisplay.Text;
            int n = str1.Length;
            txtDisplay.Text = (str1.Substring(0, n - 1));
        }
        private void btnC_Click(object sender, EventArgs e)
        {
            txtDisplay.Text = string.Empty;
            textBox1.Text = string.Empty;
        }
        private void btnCE_Click(object sender, EventArgs e)
        {
            txtDisplay.Text = "0";
            textBox1.Text = string.Empty;
        }
        // dragg form
        public
        const int WM_NCLBUTTONDOWN = 0xA1;
        public
        const int HT_CAPTION = 0x2;
        [DllImportAttribute("user32.dll")]
        public static extern int SendMessage(IntPtr hWnd, int Msg, int wParam, int lParam);
        [DllImportAttribute("user32.dll")]
        public static extern bool ReleaseCapture();
        private void topPanel_MouseDown(object sender, MouseEventArgs e)
        {
            ReleaseCapture();
            SendMessage(this.Handle, WM_NCLBUTTONDOWN, HT_CAPTION, 0);
        }
        private void btn1x_Click(object sender, EventArgs e)
        {
            result = 1 / decimal.Parse(txtDisplay.Text);
            textBox1.Text = 1 + " / " + txtDisplay.Text + " ";
            txtDisplay.Text = result.ToString();
        }
        private void btnV_Click(object sender, EventArgs e)
        {
            double num = Math.Sqrt(double.Parse(txtDisplay.Text));
            textBox1.Text = "√ " + txtDisplay.Text;
            result = Convert.ToDecimal(num);
            txtDisplay.Text = result.ToString();
        }
        private void btnClose_Click(object sender, EventArgs e)
        {
            Application.Exit();
        }
        private void btnMin_Click(object sender, EventArgs e)
        {
            this.WindowState = FormWindowState.Minimized;
        }
        //..
    }
}
//Author: Bruno Kiafuka