CalcStart

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 Calculator
{
    public partial class Form1 : Form
    {
        double a, b, result = 0;
        char operation;

        public Form1()
        {
            InitializeComponent();
        }

        private void btnEq_Click(object sender, EventArgs e)
        {
            b = int.Parse(lblScreen.Text);
            lblScreen.Text = "";

            switch (operation)
            {
                case '+':
                    result = a + b;
                    break;
                case '-':
                    result = a - b;
                    break;
                case '*':
                    result = a * b;
                    break;
                case '/':
                    if (b != 0)
                        result = a / b;
                    else
                        MessageBox.Show("Don't devide by 0!");
                    break;

            }

            //lblScreen.Text = result.ToString();
            lblScreen.Text = $"{result}";
        }

        private void btn1_Click(object sender, EventArgs e)
        {
            Button button = (Button)sender;
            lblScreen.Text += button.Text;
        }

        private void btnPlus_Click(object sender, EventArgs e)
        {
            Button button = (Button)sender;
            a = int.Parse(lblScreen.Text);
            lblScreen.Text = "";
            operation = char.Parse(button.Text);
        }
    }
}